**3.3. [PVA-Ni(II)]n complex**

These polymer metal complexes were prepared by using template method. A solution of polyvinyl alcohol is dissolved in water and the given solution was stirred magnetically and was heated at specific temperature on hot plate. One mole of metal chloride was diffused in water; this mixture was added dropwise into solution of polymer, again stirred and heated on hot plate approximately for 1 h. Complexes were precipitated by using acetone and filtered, then washed with acetone and then dried in oven. Complexes of polyvinyl alcohol are not soluble in water. Elemental analysis showed polymer and metal ratio of about 126:1. These complexes are not prepared in the form of tablets and pellets due to its springy nature [18, 19]. These complex polymers are diamagnetic and show square planar geometry [20]. No electronic bands were formed due to insolubility of these polymers in water; these metal complex polymers show rubber-like structure. This polymer exhibited about 17% of rubber naturally. Due to its rubber-like structure, stress-strain experiments of these complex materials were performed. This complex polymer molecule has breaking strain value of about 83%. These stress-strain values are due to hydroxyl group (**Figure 3**).

**Figure 3.** Structure of PVA-metal complex polymer.

**Figure 2.** Three-dimensional diagram of complex {[Cu(3,4-Hpdc)2

DMSO with different symmetry codes showing distorted octahedral geometry.

 (H2 O)2

Due to presence of repulsion, this complex showed sawhorse geometry instead of tetrahedral

of almost same energy of same shell are involved, such type of complexes are known as spin paired complexes. Magnetic moment indicates that this complex is diamagnetic in nature because all electrons present in d orbital are paired. After intermixing of these orbitals, this

is monoclinic. To synthesize this complex, 3,4-pyridinedicorboxlic acid was dissolved in DMSO,

analysis shows that ratio of metal and ligand is 1:2 in this complex [12]. The resultant mixture becomes green; after 3 weeks, the color of the solution has changed to blue. Blue crystals were suitable for XRD analysis; these crystals were collected by filtration and washed by using DMSO. XRD analysis indicates that crystals which are formed are monoclinic in nature with space group P21/n. In given complex, central metal atom is coordinated to two nitrogen and two oxygen atoms of 3,4-Hpdc− ligands and two molecules of water leads to six coordination with octahedral geometry. Each 3,4-Hpdc− molecule is deprotonated partially, with only one carboxylate ion involving in coordination toward metal center due to the presence of vibrational frequency of CO and (COO−). No any basic material was added into reaction mixture and the reaction was carried out under optimum conditions to obtain partially protonated material. The bond

(2.006(2) Å) [13, 14]. In two-dimensional sheet of polymer, Cu…Cu distance is about 8.781 Å [15]. Thermal analysis indicates that this complex polymer molecule has coordinated molecules of water and DMSO molecule in the resulting network. Two hydrogen bonds involved in

(2.467(3) Å) is much larger than that of Cu-O4

hybrid orbitals.

**]·2dmso]n**

 **(H2 O)2**

Formula weight of this compound is 588.05 g M−<sup>1</sup>

and this mixture was added in ethanol solution of CuCl2

70 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

hybridization in which one d orbital, one s orbital, and three p orbitals

and density is 1.607 g cm−<sup>3</sup>

and Cu-N1ii

.6H2

. Its crystal system

are (1.977(2) Å) and

O by diffusion method. Elemental

geometry having dsp3

complex gives five dsp<sup>3</sup>

**3.2. [[Cu(3,4-Hpdc)2**

distance of Cu-O5

]·2dmso}n with only one lattice molecule of

This complex polymer is square planar polymer resulting from dsp2 hybridization; in this, one d, one s, and two p orbitals of same energy of same shell are involved; charge on central metal atom is Ni2+. Magnetic moment of this polymer complex indicated that this complex polymer is diamagnetic because all electrons are paired in 3d orbitals of central metal atom. Color of this complex is green. Polyvinyl alcohol donates two electrons to one of dsp2 hybrid orbitals [20].

is diamagnetic due to the presence of paired electrons in 3d orbitals. Color of this complex

In this polymer complex, ligand is Schiff base ((E)-2-((pyridin-2-yl)methyleneamino)-5 chlorobenzoic acid) which was prepared by reflux condensation of 0.536 g, 5 mmol of 2 pyridinecarboxaldehyde, and 0.858 g, 5 mmol of 2-amino-5-chlorobenzoic acid in 50 ml solution of methanol for 1 h approximately. The resulting solution is orange red in color. Performed TLC of this solution indicates the presence of Schiff base ligand. This product was separated by performing column chromatography by using mixture of ethyl acetate and light

The copper complex with this ligand was prepared by using 10 ml of methanolic solution; 0.261 g, 1 mmol of Schiff base was added to another methanolic solution of 0.290 g, 1 mmol of copper trifluoroacetate with slow stirring on hot plate for almost half an hour. The resulting mixture was blue in color and was filtered; the filtrate was kept undisturbed for 7 days. A

In this complex, every cupric center is penta-coordinated in a distorted square planar geometry, where Schiff base ligands act as tridentate ligand toward one cupric center; in fact, they are tetradentate ligand. The basal plane of Cu1 is provided by nitrogen of pyridine and imine

**Figure 5.** This figure showed the ORTEP view of asymmetric unit of complex polymer. Its bond lengths are described

hybrid

73

Coordination Chemistry of Networking Materials http://dx.doi.org/10.5772/intechopen.80864

polymer is white. O and N atoms of ligand can donate two electrons to one of four sp3

petroleum in a ratio of 1:1. Then, evaporation of this content yields pure ligand [22].

plate-shaped blue-colored crystal of this complex was obtained.

orbitals [21].

below.

**3.5. ([Cu(L)(CF3**

**COO)]2 )n**

### **3.4. [Zn(II)(SEPCPU)]n**

The name of ligand in this complex polymer is sebacoylbis-p-chlorophenyl urea. This ligand was prepared by condensation of 0.1 mmol of sebacoyl dichloride (prepared by sebacic acid and double-distilled thionyl chloride) and 0.2 mmol of p-chlorophenyl urea (prepared by p-chloroaniline and glacial acetic acid and sodium cyanate solution) in sodium-dried benzene for almost 5 h. Coordination polymer by using this ligand was formed by mixing ligand and metal acetate in minimum amount of hot dimethylformamide separately. Both solutions of ligand and metal acetate were filtered and then mixed under hot conditions. Reaction mixture was refluxed on oil bath for 5–6 h at 135–145°C. The colored products obtained were filtered and washed first with hot DMF and then with ethanol and acetone for elimination of unreacted reactants if present and finally dried in oven. This complex polymer is insoluble in water completely. Normal method of characterization is proton NMR and electronic spectra cannot be attained in solution.

IR sharp band appears at 1656 cm−<sup>1</sup> due to C〓O stretching vibrations; this band disappears in coordination polymers due to enolization and coordination of metal atom. This coordination polymer is diamagnetic in nature and shows tetrahedral geometry. This complex polymer is white in color. All coordination polymers are thermally stable and show insolubility in all organic solvents and due to their thermal stability, they can be used as powder coating materials (**Figure 4**).

This complex is tetrahedral resulted from sp3 hybridization. In this type of hybridization, one s and three p orbitals of same shell having same energy are involved; charge on central metal atom is Zn2+. Magnetic moment of this complex polymer indicates that this complex

**Figure 4.** Structure of [Zn(II)(SEPCPU)]n complex polymer.

is diamagnetic due to the presence of paired electrons in 3d orbitals. Color of this complex polymer is white. O and N atoms of ligand can donate two electrons to one of four sp3 hybrid orbitals [21].

#### **3.5. ([Cu(L)(CF3 COO)]2 )n**

**Figure 4.** Structure of [Zn(II)(SEPCPU)]n complex polymer.

This complex polymer is square planar polymer resulting from dsp2

green. Polyvinyl alcohol donates two electrons to one of dsp2

72 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

**3.4. [Zn(II)(SEPCPU)]n**

cannot be attained in solution.

materials (**Figure 4**).

IR sharp band appears at 1656 cm−<sup>1</sup>

This complex is tetrahedral resulted from sp3

one s, and two p orbitals of same energy of same shell are involved; charge on central metal atom is Ni2+. Magnetic moment of this polymer complex indicated that this complex polymer is diamagnetic because all electrons are paired in 3d orbitals of central metal atom. Color of this complex is

The name of ligand in this complex polymer is sebacoylbis-p-chlorophenyl urea. This ligand was prepared by condensation of 0.1 mmol of sebacoyl dichloride (prepared by sebacic acid and double-distilled thionyl chloride) and 0.2 mmol of p-chlorophenyl urea (prepared by p-chloroaniline and glacial acetic acid and sodium cyanate solution) in sodium-dried benzene for almost 5 h. Coordination polymer by using this ligand was formed by mixing ligand and metal acetate in minimum amount of hot dimethylformamide separately. Both solutions of ligand and metal acetate were filtered and then mixed under hot conditions. Reaction mixture was refluxed on oil bath for 5–6 h at 135–145°C. The colored products obtained were filtered and washed first with hot DMF and then with ethanol and acetone for elimination of unreacted reactants if present and finally dried in oven. This complex polymer is insoluble in water completely. Normal method of characterization is proton NMR and electronic spectra

coordination polymers due to enolization and coordination of metal atom. This coordination polymer is diamagnetic in nature and shows tetrahedral geometry. This complex polymer is white in color. All coordination polymers are thermally stable and show insolubility in all organic solvents and due to their thermal stability, they can be used as powder coating

one s and three p orbitals of same shell having same energy are involved; charge on central metal atom is Zn2+. Magnetic moment of this complex polymer indicates that this complex

hybridization; in this, one d,

hybrid orbitals [20].

due to C〓O stretching vibrations; this band disappears in

hybridization. In this type of hybridization,

In this polymer complex, ligand is Schiff base ((E)-2-((pyridin-2-yl)methyleneamino)-5 chlorobenzoic acid) which was prepared by reflux condensation of 0.536 g, 5 mmol of 2 pyridinecarboxaldehyde, and 0.858 g, 5 mmol of 2-amino-5-chlorobenzoic acid in 50 ml solution of methanol for 1 h approximately. The resulting solution is orange red in color. Performed TLC of this solution indicates the presence of Schiff base ligand. This product was separated by performing column chromatography by using mixture of ethyl acetate and light petroleum in a ratio of 1:1. Then, evaporation of this content yields pure ligand [22].

The copper complex with this ligand was prepared by using 10 ml of methanolic solution; 0.261 g, 1 mmol of Schiff base was added to another methanolic solution of 0.290 g, 1 mmol of copper trifluoroacetate with slow stirring on hot plate for almost half an hour. The resulting mixture was blue in color and was filtered; the filtrate was kept undisturbed for 7 days. A plate-shaped blue-colored crystal of this complex was obtained.

In this complex, every cupric center is penta-coordinated in a distorted square planar geometry, where Schiff base ligands act as tridentate ligand toward one cupric center; in fact, they are tetradentate ligand. The basal plane of Cu1 is provided by nitrogen of pyridine and imine

**Figure 5.** This figure showed the ORTEP view of asymmetric unit of complex polymer. Its bond lengths are described below.

and one carboxylate oxygen atom and one oxygen atom of monodentate triflouroacetate group and apical position of the complex are occupied by symmetry-related ligand. In the same way, the basal plane of Cu2 is clocked up by N4 , N3 , O5 , O7 and apical position is clocked up by O2 atom of Schiff base coordinated to Cu1 (**Figure 5**).

IR study and electronic spectra indicated that dimethylglyoxime and N-acetylglycine were

All metal complexes are insoluble in water as well as in most organic solvents but soluble in DMF and DMSO. They are nonelectrolytes. The electronic spectra of this complex indicate

system showing absorption bands at 445 and 514 nm in visible region showing electronic

and two p orbitals of same shell having same energy are involved. Charge on central metal atom is Ni2+. Its magnetic moment indicates that two electrons of five 3d orbital are unpaired which become paired by using strong ligands such as dimethylglyoxime and N-acetylglycine; as a result, complex becomes diamagnetic in character and shows pink color. Due to dsp<sup>2</sup>

moment, molecular weight of complex and its conductivity measurements show that such type of complexes is diamagnetic and monomeric. Compounds of Os and Ru are isomorphs of each other and their crystal structure is monoclinic. This complex can be distorted square

This complex molecule consists of only 68% by weight of carbon; so the structure of this

 are 2.374, 2.412, 2.230, 2.387, and 2.388 Å, respectively. In present case, vacant octahedral site of complex is occupied by phenyl ring especially by phenyl hydrogen. Ru and Os complexes of Vaska are slightly soluble in most solvents. In this complex, color change occurs

g, etc. Such type of transitions of these complexes indicates that structure

with triphenylphosphine in 2-methoxyethanol at 25°C. From magnetic

hybridization. In this type of hybridization, one d, one s,

Coordination Chemistry of Networking Materials http://dx.doi.org/10.5772/intechopen.80864

system. This complex was prepared by Vaska by

, Ru-P2

, in which only hydrogen atoms are shown which block the unused octahedral

, Ru-P3

, Ru-Cl1

, and

75

coordinated to metal by N and O atoms (**Figure 6**).

hybridization, its geometry is square planar [24]

pyramidal when Ru occupied the position of center of gravity.

complex is based on Ru, P, and Cl atoms. Bond distances of Ru-P1

g-A2

This complex is resulted from dsp2

of this complex is square planar.

d8

transitions at A1

**4.2. RuCl2**

using (NH4

Ru-Cl2

**[P(C6 H5 )3 ] 3** This complex is five-coordinated and d<sup>6</sup>

> )2 RuBr6

**Figure 7.** The structure of RuCl2

position.

[P(C6 H5 )3 ] 3

Bond lengths in this complex molecule are Cu1 -O1 , Cu2 -O2 , Cu1 -O3 , Cu2 -O5 , Cu1 -N1 , Cu2 -O7 , Cu1 -N2 , Cu2 -N3 , Cu1 -O6 , and Cu2 -N4 1.904, 2.162, 1.941, 1.895, 2.011, 1.939, 2.006, 2.017, 2.178, 2.008 Å, respectively [23].

The geometry of this complex polymer is square pyramidal. Hybridization of this polymer is sp3 d2 which involves one s and three p orbitals of same shell and outer two d orbitals of fourth shell. Charge on central metal atom is Cu2+; its magnetic moment indicates that this complex polymer is paramagnetic in behavior because one unpaired electron was left in one of 3d orbitals. And ligands show interactions toward metal center through sp3 d2 orbitals by donation of two electrons.

### **4. Examples of transition metal compounds**

#### **4.1. [Ni(D)(G)]: D = dimethylglyoxime, G = N-acetylglycine**

An ethanolic solution of potassium hydroxide of dimethylglyoxime and of N-acetylglycine was added to aqueous solution of metal salts after some stirring on water bath; precipitates of product are formed and immediately filtered and washed by mixture of ethanol and water in ratio of 1:3, and finally dried in oven at 60°C. The metal complexes which are formed are solids and completely insoluble in organic solvents showing complete solubility in DMF. IR spectra of DMG showed absorption bands at 3400, 2931, 1570, 1141, and 756 cm−<sup>1</sup> which are accounted for ν(OH), ν(C▬H), ν(C〓N), ν(N**▬**O), and ν(C〓N**▬**O), respectively. In metal ligand complexes, these bands shifted toward lower frequencies.

IR band of N-acetylglycine was appeared at 3380 cm−<sup>1</sup> ; on complexation, this band shifted toward lower value. Electronic spectra of Ni(II) complex show absorption bands at 445 nm.

**Figure 6.** Square planar structure of the complex.

IR study and electronic spectra indicated that dimethylglyoxime and N-acetylglycine were coordinated to metal by N and O atoms (**Figure 6**).

All metal complexes are insoluble in water as well as in most organic solvents but soluble in DMF and DMSO. They are nonelectrolytes. The electronic spectra of this complex indicate d8 system showing absorption bands at 445 and 514 nm in visible region showing electronic transitions at A1 g-A2 g, etc. Such type of transitions of these complexes indicates that structure of this complex is square planar.

This complex is resulted from dsp2 hybridization. In this type of hybridization, one d, one s, and two p orbitals of same shell having same energy are involved. Charge on central metal atom is Ni2+. Its magnetic moment indicates that two electrons of five 3d orbital are unpaired which become paired by using strong ligands such as dimethylglyoxime and N-acetylglycine; as a result, complex becomes diamagnetic in character and shows pink color. Due to dsp<sup>2</sup> hybridization, its geometry is square planar [24]

#### **4.2. RuCl2 [P(C6 H5 )3 ] 3**

**Figure 6.** Square planar structure of the complex.

and one carboxylate oxygen atom and one oxygen atom of monodentate triflouroacetate group and apical position of the complex are occupied by symmetry-related ligand. In the

> , N3 , O5 , O7

1.904, 2.162, 1.941, 1.895, 2.011, 1.939, 2.006, 2.017, 2.178,


which involves one s and three p orbitals of same shell and outer two d orbitals of

The geometry of this complex polymer is square pyramidal. Hybridization of this polymer

fourth shell. Charge on central metal atom is Cu2+; its magnetic moment indicates that this complex polymer is paramagnetic in behavior because one unpaired electron was left in one

An ethanolic solution of potassium hydroxide of dimethylglyoxime and of N-acetylglycine was added to aqueous solution of metal salts after some stirring on water bath; precipitates of product are formed and immediately filtered and washed by mixture of ethanol and water in ratio of 1:3, and finally dried in oven at 60°C. The metal complexes which are formed are solids and completely insoluble in organic solvents showing complete solubility in DMF. IR

accounted for ν(OH), ν(C▬H), ν(C〓N), ν(N**▬**O), and ν(C〓N**▬**O), respectively. In metal

toward lower value. Electronic spectra of Ni(II) complex show absorption bands at 445 nm.

spectra of DMG showed absorption bands at 3400, 2931, 1570, 1141, and 756 cm−<sup>1</sup>

of 3d orbitals. And ligands show interactions toward metal center through sp3

and apical position is clocked

d2

orbitals by

which are

; on complexation, this band shifted

same way, the basal plane of Cu2 is clocked up by N4

74 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

, and Cu2

**4. Examples of transition metal compounds**

**4.1. [Ni(D)(G)]: D = dimethylglyoxime, G = N-acetylglycine**

ligand complexes, these bands shifted toward lower frequencies.

IR band of N-acetylglycine was appeared at 3380 cm−<sup>1</sup>

Bond lengths in this complex molecule are Cu1

atom of Schiff base coordinated to Cu1 (**Figure 5**).


up by O2

2.008 Å, respectively [23].

donation of two electrons.

Cu1 -N2 , Cu2 -N3 , Cu1 -O6

is sp3 d2

> This complex is five-coordinated and d<sup>6</sup> system. This complex was prepared by Vaska by using (NH4 )2 RuBr6 with triphenylphosphine in 2-methoxyethanol at 25°C. From magnetic moment, molecular weight of complex and its conductivity measurements show that such type of complexes is diamagnetic and monomeric. Compounds of Os and Ru are isomorphs of each other and their crystal structure is monoclinic. This complex can be distorted square pyramidal when Ru occupied the position of center of gravity.

> This complex molecule consists of only 68% by weight of carbon; so the structure of this complex is based on Ru, P, and Cl atoms. Bond distances of Ru-P1 , Ru-P2 , Ru-P3 , Ru-Cl1 , and Ru-Cl2 are 2.374, 2.412, 2.230, 2.387, and 2.388 Å, respectively. In present case, vacant octahedral site of complex is occupied by phenyl ring especially by phenyl hydrogen. Ru and Os complexes of Vaska are slightly soluble in most solvents. In this complex, color change occurs

**Figure 7.** The structure of RuCl2 [P(C6 H5 )3 ] 3 , in which only hydrogen atoms are shown which block the unused octahedral position.

due to rotation of phenyl ring. Preferable geometry for this complex is square pyramidal or octahedral. But stability of this complex may arise by blocking of unused octahedral site by rotation of phenyl ring [25] (**Figure 7**).

The hybridization of this complex is sp3

square planar in geometry having sp3

pyramid. Reaction of [Cu(acac)(Me2

**4.4. [Cu(acac)(Me2**

bipy)](ClO4

(Me2

ligands.

of metal atom and exhibited tetrahedral geometry.

**bipy)(NCS)]**

Ligands have been prepared by dissolving 2 mmol Me<sup>2</sup>

added over 25 ml of aqueous solution of 2 mmol Cu(ClO4

tion of this mixture gives brown-colored crystals of complex [Cu(acac)(Me2

bond angles of O-Cu-O and N-Cu-N are 94.74(11) and 81.44(11), respectively.

bipy)](ClO4

, in which one s and three p orbitals are involved.

hybridization. Perchlorate ion is not involved in geometry.

bipy in 20 ml of ethanol and other ligand

Coordination Chemistry of Networking Materials http://dx.doi.org/10.5772/intechopen.80864

rapidly. The slow process of evapora-

) and KCNS gives mononuclear complex with

bipy)](ClO4

) which is

77

Magnetic measurement of this complex indicates that this complex is diamagnetic because its all d orbital electrons are paired, electrons lose only from s orbital of Zn due to charge on Zn2+ and no magnetic moment was observed and ligand attaches to metal atom via s and p orbitals

has been prepared by 2 mmol acetylacetone. Stoichiometric quantity of Hacac on LiOH was

The above complex has been prepared by reaction of ethanolic solution of 0.5 mmol [Cu(acac)

evaporation of this mixture leads to green-colored crystals. These crystals are monoclinic form. The resultant complex has square pyramidal geometry. The bond distance of Cu▬O is shorter than that of Cu▬N such as [1.882(3); 1.896(3) Å] and s [1.981(3); 1.984(3) Å] and the

The insertion of another ligand to complex of Cu which acts as bridging ligand leads to square

isothiocyanato ligand coordinated to copper in apical position. In this complex, two ligands overlap in face-to-face manner with interplanar distances of about 3.09–3.37 Å and 3.50–3.57 Å

**Figure 9.** Crystal structure of complex [Cu(acac)(Me2bipy)(NCS)] (a) numbering scheme of atoms and its packing diagram, (b) stacking interaction among ligand molecules, and (c) stacking interaction among Me2bipy ligand and acac

) 2

) and aqueous solution of 10 ml of 0.5 mmol KCNS. The slow process of

This complex resulted from d2 sp3 hybridization. Charge on central metal atom is Ru (+2). Magnetic moment of this complex showed that electrons are paired per Ru2+ atom. The 3dx2 y2 and 3dz2 orbitals, one s, and three p orbitals of same energy of same shell are involved in d2 sp3 hybridization. Triphenylphosphine is the strong ligand force pairing of all electrons of Ru. This complex is diamagnetic in behavior and may be known as spin paired complex and forms octahedral geometry.

## **4.3. Zinc(4-amino-5-pyridyl-4H-1,2,4-triazole-3-thiol)2**

In ethanol, a mixture of isonicotinic acid from potassium hydroxide was dissolved; when fluxing or dissipation was complete, carbon disulfide was added slowly to this mixture. And then, it was stirred on hot plate for almost 10 h; dried ether was added to this content resulting in precipitates of yellow color; the precipitates are filtered and washed by using ether and dried. Then, these yellow precipitates were added into excess amount of hydrazine hydride and were refluxed along with stirring until estimation of hydrogen sulfide was done. This process was stopped by using lead acetate paper; after cooling, this mixture was filtered and then acidified by using hydrochloric acid to yield product of white color. That is the ligand used in this complex formation.

Ethanolic solution of metal salt zinc acetate dihydrate was added into ethanolic solution of ligand in metal to ligand ratio of about 1:2 and refluxed for 2 h; crystalline colored precipitates were appeared at room temperature and washed by hot methanol and left for drying and recrystallized by using ethanol.

FTIR spectra showed some specific vibrations of ligand at 3250 and 3213, 2736, 1645, 673, 529, 432 cm−<sup>1</sup> due to NH2 , S**▬**H, C〓N, M**▬**N, and M**▬**S of triazole ring and metal ligand complex, and the last one is due to stretching of C**▬**S bond, respectively. Tautomeric form of triazole could be responsible for deprotonation of ligand before complexation; after complexation, ligand can attach to metal ion either through N atom or through S atom of thioamide group; bonding at S atom is more preferable because this gives more stable chelate. Electronic spectra of ligand exhibit three bands at 263, 302, 309 nm due to (π-π\*), (π-π\*), (n-π\*) intraligand transitions. The given complex is diamagnetic in behavior due to completely filled d-orbitals, so no any d-d transitions can be possible in visible region [26] (**Figure 8**).

**Figure 8.** Structure of zinc(4-amino-5-pyridyl-4H-1,2,4-triazole-3-thiol)2 complex.

The hybridization of this complex is sp3 , in which one s and three p orbitals are involved. Magnetic measurement of this complex indicates that this complex is diamagnetic because its all d orbital electrons are paired, electrons lose only from s orbital of Zn due to charge on Zn2+ and no magnetic moment was observed and ligand attaches to metal atom via s and p orbitals of metal atom and exhibited tetrahedral geometry.

#### **4.4. [Cu(acac)(Me2 bipy)(NCS)]**

**Figure 8.** Structure of zinc(4-amino-5-pyridyl-4H-1,2,4-triazole-3-thiol)2

complex.

, S**▬**H, C〓N, M**▬**N, and M**▬**S of triazole ring and metal ligand

due to rotation of phenyl ring. Preferable geometry for this complex is square pyramidal or octahedral. But stability of this complex may arise by blocking of unused octahedral site by

Magnetic moment of this complex showed that electrons are paired per Ru2+ atom. The 3dx2

In ethanol, a mixture of isonicotinic acid from potassium hydroxide was dissolved; when fluxing or dissipation was complete, carbon disulfide was added slowly to this mixture. And then, it was stirred on hot plate for almost 10 h; dried ether was added to this content resulting in precipitates of yellow color; the precipitates are filtered and washed by using ether and dried. Then, these yellow precipitates were added into excess amount of hydrazine hydride and were refluxed along with stirring until estimation of hydrogen sulfide was done. This process was stopped by using lead acetate paper; after cooling, this mixture was filtered and then acidified by using hydrochloric acid to yield product of white color. That is the ligand used in this complex formation.

Ethanolic solution of metal salt zinc acetate dihydrate was added into ethanolic solution of ligand in metal to ligand ratio of about 1:2 and refluxed for 2 h; crystalline colored precipitates were appeared at room temperature and washed by hot methanol and left for drying and

FTIR spectra showed some specific vibrations of ligand at 3250 and 3213, 2736, 1645, 673,

complex, and the last one is due to stretching of C**▬**S bond, respectively. Tautomeric form of triazole could be responsible for deprotonation of ligand before complexation; after complexation, ligand can attach to metal ion either through N atom or through S atom of thioamide group; bonding at S atom is more preferable because this gives more stable chelate. Electronic spectra of ligand exhibit three bands at 263, 302, 309 nm due to (π-π\*), (π-π\*), (n-π\*) intraligand transitions. The given complex is diamagnetic in behavior due to completely filled d-orbitals,

so no any d-d transitions can be possible in visible region [26] (**Figure 8**).

orbitals, one s, and three p orbitals of same energy of same shell are involved in

 hybridization. Triphenylphosphine is the strong ligand force pairing of all electrons of Ru. This complex is diamagnetic in behavior and may be known as spin paired complex and

hybridization. Charge on central metal atom is Ru (+2).


rotation of phenyl ring [25] (**Figure 7**).

sp3

**4.3. Zinc(4-amino-5-pyridyl-4H-1,2,4-triazole-3-thiol)2**

76 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

This complex resulted from d2

forms octahedral geometry.

recrystallized by using ethanol.

due to NH2

529, 432 cm−<sup>1</sup>

y2

d2 sp3

and 3dz2

Ligands have been prepared by dissolving 2 mmol Me<sup>2</sup> bipy in 20 ml of ethanol and other ligand has been prepared by 2 mmol acetylacetone. Stoichiometric quantity of Hacac on LiOH was added over 25 ml of aqueous solution of 2 mmol Cu(ClO4 ) 2 rapidly. The slow process of evaporation of this mixture gives brown-colored crystals of complex [Cu(acac)(Me2 bipy)](ClO4 ) which is square planar in geometry having sp3 hybridization. Perchlorate ion is not involved in geometry.

The above complex has been prepared by reaction of ethanolic solution of 0.5 mmol [Cu(acac) (Me2 bipy)](ClO4 ) and aqueous solution of 10 ml of 0.5 mmol KCNS. The slow process of evaporation of this mixture leads to green-colored crystals. These crystals are monoclinic form. The resultant complex has square pyramidal geometry. The bond distance of Cu▬O is shorter than that of Cu▬N such as [1.882(3); 1.896(3) Å] and s [1.981(3); 1.984(3) Å] and the bond angles of O-Cu-O and N-Cu-N are 94.74(11) and 81.44(11), respectively.

The insertion of another ligand to complex of Cu which acts as bridging ligand leads to square pyramid. Reaction of [Cu(acac)(Me2 bipy)](ClO4 ) and KCNS gives mononuclear complex with isothiocyanato ligand coordinated to copper in apical position. In this complex, two ligands overlap in face-to-face manner with interplanar distances of about 3.09–3.37 Å and 3.50–3.57 Å

**Figure 9.** Crystal structure of complex [Cu(acac)(Me2bipy)(NCS)] (a) numbering scheme of atoms and its packing diagram, (b) stacking interaction among ligand molecules, and (c) stacking interaction among Me2bipy ligand and acac ligands.

for acac and Me2 bipy ligands, respectively and give dimmers. Within dimer structures, distance between Cu(II) ions is 3.920 Å [27] (**Figure 9**).

that of Co-EDTA complex. The bond lengths of Co(1)-O(1), Co(1)-O(5), Co(1)-O(3), Co(1)- O(1W), Co(1)-O(7), Co(1)-N(2), Co(1)-N(1) are 2.124, 2.272, 2.465, 2.073, 2.078, 2.229, and 2.256 Å, respectively. Bond lengths and bond angles suggest that geometry of this complex is

in pentagonal bipyramidal symmetry. Charge on central metal atom is Co (+3); its magnetic moment indicates that this complex is diamagnetic in behavior because all electrons present in 3d orbitals become paired favored by EDTA ligand. Ligand showed interaction toward central metal atom via one d, one s, and three p orbitals of third shell and two d(x2−y2) and dz2

This chapter is focused on coordination chemistry of metal organic frameworks and compounds of transition metals. Bonding and hybridization in these compounds was explained by valence bond theory and molecular orbital theory; specific distortions due to the presence of lone pair of electrons were explained via crystal field theory. Magnetic behavior, color of compounds described above, space groups, crystal shapes, and geometry of the complex

and Amin Badshah2

2 Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan

[1] Zhang Y. On study of new progress and application of coordination chemistry in chemistry and chemical industry in recent years. In: IOP Conference Series: Earth and Environ-

[2] Bowman-James K. Alfred Werner revisited: The coordination chemistry of anions.

[3] Busch DH. The complete coordination chemistry-one practioner's perspective. Chemical

d2

due to involvement of one s, three p,

Coordination Chemistry of Networking Materials http://dx.doi.org/10.5772/intechopen.80864

which are arranged

79

d2

pentagonal bipyramidal [28] (**Figure 10**).

resulting in seven dsp3

compounds were also explained.

\*, Sumbal Naz1

mental Science. IOP Publishing; 2017

Reviews. 1993;**93**(3):847-860

Accounts of Chemical Research. 2005;**38**(8):671-678

\*Address all correspondence to: atafali.altaf@uog.edu.pk

1 Department of Chemistry, University of Gujrat, Gujrat, Pakistan

**5. Conclusion**

**Author details**

Ataf Ali Altaf1

**References**

The hybridization of this complex molecule is dsp3

d2

and three d orbitals intermixed to give seven hybrid orbitals of dsp3

hybrid orbitals.

The hybridization of this complex is sp3 d2 resulted by the involvement of one s, three p, and two d orbitals. Magnetic moment of this complex indicates that this complex is diamagnetic because all electrons are paired in 3d orbitals; electrons are lost from 3s orbital due to Cu2+ charge. The ligands are coordinated toward metal ion via one s, three p, and two 4d orbitals leading to square pyramidal geometry.

#### **4.5. [Co(H2 O)(EDTAH)].2H2 O**

A 0.291 g of Co(No3 )2 .6H2 O was mixed with 0.416 g of sodium salt of EDTA in distilled deionized water at room temperature. On disintegration of these two reactants, 3% solution of hydrogen peroxide was added with constant stirring until changing in color was complete. The volume of water reduced slowly by evaporation over a period of several days. Crystals of compound were left. Purity of product and quality of crystals depend on the rate of reaction. The fast rate of reaction would lead to the impurity formation and mixture of products.

The structure of this complex is distorted pentagonal bipyramidal. Cobalt is attached to nitrogen atom and oxygen atoms of three acetate groups and one acetic acid group of EDTAH−<sup>3</sup> and one oxygen atom of water molecule. Axial positions of the complex were being occupied by two oxygen atoms of different acetate groups, and pentagonal plane of this molecule was being occupied by two nitrogen donors, one oxygen of EDTAH−<sup>3</sup> , one oxygen atom of acetato group, and one oxygen atom of water molecule.

Bond distance of carbon and oxygen for protonated O(4) is 1.294 Å. The bond distance of coordinated O(3)-C(4) is 1.215 Å. Bond lengths of attached ligand molecule are longer than

**Figure 10.** Structure of Co(EDTAH)(H<sup>2</sup> O) complex which indicates the distortion in pentagonal plane.

that of Co-EDTA complex. The bond lengths of Co(1)-O(1), Co(1)-O(5), Co(1)-O(3), Co(1)- O(1W), Co(1)-O(7), Co(1)-N(2), Co(1)-N(1) are 2.124, 2.272, 2.465, 2.073, 2.078, 2.229, and 2.256 Å, respectively. Bond lengths and bond angles suggest that geometry of this complex is pentagonal bipyramidal [28] (**Figure 10**).

The hybridization of this complex molecule is dsp3 d2 due to involvement of one s, three p, and three d orbitals intermixed to give seven hybrid orbitals of dsp3 d2 which are arranged in pentagonal bipyramidal symmetry. Charge on central metal atom is Co (+3); its magnetic moment indicates that this complex is diamagnetic in behavior because all electrons present in 3d orbitals become paired favored by EDTA ligand. Ligand showed interaction toward central metal atom via one d, one s, and three p orbitals of third shell and two d(x2−y2) and dz2 resulting in seven dsp3 d2 hybrid orbitals.

## **5. Conclusion**

for acac and Me2

**4.5. [Co(H2**

A 0.291 g of Co(No3

tance between Cu(II) ions is 3.920 Å [27] (**Figure 9**).

78 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

**O**

being occupied by two nitrogen donors, one oxygen of EDTAH−<sup>3</sup>

group, and one oxygen atom of water molecule.

**Figure 10.** Structure of Co(EDTAH)(H<sup>2</sup>

The hybridization of this complex is sp3

leading to square pyramidal geometry.

**O)(EDTAH)].2H2**

) 2 .6H2

bipy ligands, respectively and give dimmers. Within dimer structures, dis-

O was mixed with 0.416 g of sodium salt of EDTA in distilled deion-

resulted by the involvement of one s, three p, and

, one oxygen atom of acetato

d2

two d orbitals. Magnetic moment of this complex indicates that this complex is diamagnetic because all electrons are paired in 3d orbitals; electrons are lost from 3s orbital due to Cu2+ charge. The ligands are coordinated toward metal ion via one s, three p, and two 4d orbitals

ized water at room temperature. On disintegration of these two reactants, 3% solution of hydrogen peroxide was added with constant stirring until changing in color was complete. The volume of water reduced slowly by evaporation over a period of several days. Crystals of compound were left. Purity of product and quality of crystals depend on the rate of reaction. The fast rate of reaction would lead to the impurity formation and mixture of products.

The structure of this complex is distorted pentagonal bipyramidal. Cobalt is attached to nitrogen atom and oxygen atoms of three acetate groups and one acetic acid group of EDTAH−<sup>3</sup> and one oxygen atom of water molecule. Axial positions of the complex were being occupied by two oxygen atoms of different acetate groups, and pentagonal plane of this molecule was

Bond distance of carbon and oxygen for protonated O(4) is 1.294 Å. The bond distance of coordinated O(3)-C(4) is 1.215 Å. Bond lengths of attached ligand molecule are longer than

O) complex which indicates the distortion in pentagonal plane.

This chapter is focused on coordination chemistry of metal organic frameworks and compounds of transition metals. Bonding and hybridization in these compounds was explained by valence bond theory and molecular orbital theory; specific distortions due to the presence of lone pair of electrons were explained via crystal field theory. Magnetic behavior, color of compounds described above, space groups, crystal shapes, and geometry of the complex compounds were also explained.

### **Author details**

Ataf Ali Altaf1 \*, Sumbal Naz1 and Amin Badshah2


### **References**


[4] Werner A. Neuere Anschauungen auf dem Gebiete der anorganischen chemie. Vol. 8. F. Vieweg und Sohn; 1920

[19] Arafa I, El-Ghanem H, Al-Shalabi R. Formation, characterization and electrical conductivity of polycarbosilazane-Cu(II),-Ni(II) and-Cr(III) chloride metallopolymers. Journal

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[20] Sari N et al. Synthesis of some polymer-metal complexes and elucidation of their structures. Journal of Macromolecular Science Part A: Pure and Applied Chemistry. 2006;

[21] Bonde A et al. Synthesis, spectral and thermal studies of coordination polymers of sebacoyl bis-P-chlorophenyl urea. RASAYAN Journal of Chemistry. 2011;**4**(4):838-843

[22] Shit S et al. Crystal structure, characterization and magnetic properties of a 1D copper (II) polymer incorporating a Schiff base with carboxylate side arm. Journal of Chemical

[23] Shit S et al. Syntheses, structural variations and fluorescence studies of two dinuclear zinc (II) complexes of a Schiff base ligand with an extended carboxylate side arm. Journal

[24] Shaker SA. Preparation and spectral properties of mixed-ligand complexes of VO(IV), Ni(II), Zn(II), Pd(II), Cd(II) and Pb(II) with dimethylglyoxime and N-acetylglycine. Journal

[25] La Placa SJ, Ibers JA. A five-coordinated d6 complex: Structure of dichlorotris (triphe-

[26] Haddad R, Yousif E, Ahmed A. Synthesis and characterization of transition metal complexes of 4-amino-5-pyridyl-4H-1,2,4-triazole-3-thiol. Springerplus. 2013;**2**(1):510 [27] Madalan AM et al. Chemistry at the apical position of square-pyramidal copper (II) complexes: Synthesis, crystal structures, and magnetic properties of mononuclear Cu(II), and heteronuclear Cu(II)-Hg(II) and Cu(II)-Co(II) complexes containing [Cu (AA)(BB)] + moieties (AA = acetylacetonate, salicylaldehydate; BB = 1,10-phenanthroline, Me 2 bipy =

4,4′-dimethyl-2,2′-bipyridine). Inorganica Chimica Acta. 2004;**357**(14):4151-4164

[28] Zubkowski JD et al. A seven coordinate co-EDTA complex. Crystal and molecular structure of aquo (ethylenediaminetriacetatoacetic acid) cobalt (III) dihydrate. Inorganic Chemistry.

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1995;**34**(25):6409-6411


[19] Arafa I, El-Ghanem H, Al-Shalabi R. Formation, characterization and electrical conductivity of polycarbosilazane-Cu(II),-Ni(II) and-Cr(III) chloride metallopolymers. Journal of Inorganic and Organometallic Polymers. 2003;**13**(2):69-86

[4] Werner A. Neuere Anschauungen auf dem Gebiete der anorganischen chemie. Vol. 8.

[5] Zaworotko MJ. Coordination polymers. NATO ASI Series C Mathematical and Physical

[6] Fujita M et al. Self-assembled molecular ladders. New Journal of Chemistry. 1998;**22**(2):

[7] Venkataraman D et al. Coordination networks based on multitopic ligands and silver (I) salts: A study of network connectivity and topology as a function of counterion. Chemistry

[9] Janiak C. Engineering coordination polymers towards applications. Dalton Transactions.

[10] Robin AY, Fromm KM. Coordination polymer networks with O-and N-donors: What they are, why and how they are made. Coordination Chemistry Reviews. 2006;**250**(15-16):

[11] Kitagawa S, Kitaura R, Noro Si. Functional porous coordination polymers. Angewandte

[12] Yan S-H et al. Self-assembly and characterization of copper 3,4-pyridinedicarboxylate complexes based on a variety of polynuclear hydroxo clusters. Dalton Transactions. 2011;**40**(8):

[13] Fu ZY et al. Three novel polymeric frameworks assembled from CdII, CoII, and MnII with the mixed organic ligands 3,4-pyridinedicarboxylate, 1,3-bis(4-pyridyl) propane, or 1,2-bis(4-pyridyl)ethane. European Journal of Inorganic Chemistry. 2003;**2003**(14):

[14] Yan S, Li X, Zheng X. Effect of the carboxyl groups on the assembly of copper pyridinedi-

[15] Blatov V. TOPOS-version 4.0 professional (Beta evaluation), Samara State University, Samara, Russia, 2006 search PubMed; (b) VA Blatov, AP Shevchenko and VN Serezhkin.

[16] Steed JW, Atwood JL. Supramolecular Chemistry. 2nd Edition. USA: John Wiley & Sons;

[17] Scaldini FM et al. 2-D coordination polymers of copper and cobalt with 3,4-pyridinedicarboxylic acid: Synthesis, characterization, and crystal structures. Journal of Coordination

[18] El-Sonbati A, Al-Shihri A, El-Bindary A. Polymer complexes. XLIII. EPR, spectra, and stereochemical versatility of novel copper (II) polymer complexes. Journal of Inorganic

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and Organometallic Polymers. 2003;**13**(2):99-108

Chemistry. 2014;**67**(18):2967-2982


**Chapter 5**

Provisional chapter

**The Triply Bonded Al≡Sb Molecules: A Theoretical**

DOI: 10.5772/intechopen.78412

The effect of substitution on the potential energy surfaces of RAl☰SbR (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*) is investigated using density functional theories (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ + dp). The theoretical results demonstrated that all the triply bonded RAl☰SbR compounds with small substituents are unstable and can spontaneously rearrange to other doubly bonded isomers. That is, the smaller groups, such as R = F, OH, H, CH3 and SiH3, neither kinetically nor thermodynamically stabilize the triply bonded RAl☰SbR compounds. However, the triply bonded R'Al☰SbR´ molecules that feature bulkier substituents (R´ = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*) are found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. In particular, the bonding characters of the R'Al☰SbR´ species agree well with the valence-electron bonding model (model) as well as several theoretical analyses (the natural bond orbital, the natural resonance theory, and the charge decomposition analysis). That is to say, R'Al☰SbR´

shows that both the electronic and the steric effects of bulkier substituent groups play a

The chemical synthesis and structural characterization of molecules that feature triple bonds [2] between heavier group 14 elements (E14 = Si, Ge, Sn and Pb) are of interest because of their interesting structural chemistry and their potential applications in organic and inorganic synthesis [1–10]. Although understanding of these RE14☰E14R molecules that feature heavier

Keywords: aluminum, antimony, group 13 elements, group 13 elements, triple bond

dAl SbdR<sup>0</sup>

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

. Their theoretical evidence

Al☰SbR<sup>0</sup> species synthetically accessible and

The Triply Bonded Al☰Sb Molecules: A Theoretical

Jia-Syun Lu, Ming-Chung Yang and Ming-Der Su

Jia-Syun Lu, Ming-Chung Yang and Ming-Der Su

Additional information is available at the end of the chapter

molecules that feature groups are regarded as R<sup>0</sup>

decisive role in making triply bonded R<sup>0</sup>

isolable in a stable form.

1. Introduction

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78412

**Prediction**

Abstract

Prediction

#### **The Triply Bonded Al≡Sb Molecules: A Theoretical Prediction** The Triply Bonded Al☰Sb Molecules: A Theoretical Prediction

DOI: 10.5772/intechopen.78412

Jia-Syun Lu, Ming-Chung Yang and Ming-Der Su Jia-Syun Lu, Ming-Chung Yang and Ming-Der Su

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78412

#### Abstract

The effect of substitution on the potential energy surfaces of RAl☰SbR (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*) is investigated using density functional theories (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ + dp). The theoretical results demonstrated that all the triply bonded RAl☰SbR compounds with small substituents are unstable and can spontaneously rearrange to other doubly bonded isomers. That is, the smaller groups, such as R = F, OH, H, CH3 and SiH3, neither kinetically nor thermodynamically stabilize the triply bonded RAl☰SbR compounds. However, the triply bonded R'Al☰SbR´ molecules that feature bulkier substituents (R´ = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*) are found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. In particular, the bonding characters of the R'Al☰SbR´ species agree well with the valence-electron bonding model (model) as well as several theoretical analyses (the natural bond orbital, the natural resonance theory, and the charge decomposition analysis). That is to say, R'Al☰SbR´ molecules that feature groups are regarded as R<sup>0</sup> dAl SbdR<sup>0</sup> . Their theoretical evidence shows that both the electronic and the steric effects of bulkier substituent groups play a decisive role in making triply bonded R<sup>0</sup> Al☰SbR<sup>0</sup> species synthetically accessible and isolable in a stable form.

Keywords: aluminum, antimony, group 13 elements, group 13 elements, triple bond

### 1. Introduction

The chemical synthesis and structural characterization of molecules that feature triple bonds [2] between heavier group 14 elements (E14 = Si, Ge, Sn and Pb) are of interest because of their interesting structural chemistry and their potential applications in organic and inorganic synthesis [1–10]. Although understanding of these RE14☰E14R molecules that feature heavier

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

group 14 atoms has increased during the last two decades, the understanding of the RE13☰E15R compounds, which are isoelectronic to acetylene from a valence electron viewpoint, is still limited. The reason for this limited knowledge of acetylene analogues, RE13☰E15R, could be due to the fact that there has been limited preparation and the isolation of these species in a stable form [11, 12]. Theoretical methods allow a theoretical design of the RE13☰E15R molecules to be made that increases understanding of their potential properties.

and B3LYP/LANL2DZ + dp are used for small substituents (R = H, F, OH, CH3, and SiH3) and M06-2X/Def2-TZVP [18] for large substituents (R = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*; see

The Triply Bonded Al≡Sb Molecules: A Theoretical Prediction

http://dx.doi.org/10.5772/intechopen.78412

85

The valence-bond bonding model is a well-known satisfactory method, which is an approximate theory to explain the electron pair or chemical bond by quantum mechanics, for predicting molecular geometries [20]. Two valence-bond bonding models (Figure 1) are thus used to interpret the bonding properties of triply bonded RAl☰SbR species. In model [1], the RAl☰SbR molecule is partitioned into two units: a singlet RdAl and a singlet RdSb. In model [2], the RAl☰SbR compound is divided into two moieties: a triplet RdAl and a triplet RdSb.

Figure 1. The valence-bond bonding models [1, 2] for the triply bonded RAl☰SbR molecule.

Scheme 1) [19].

2. General considerations

The III-V semiconductors that contain antimony have several important applications in optoelectronic devices that operate in the infrared region and in high-speed devices, which has prompted widespread studies of promising precursor systems for these materials [13]. In particular, the chemical synthesis and structural characterization of AlSb single-source precursors of the type R3Al-SbR´3 has attracted much attention, owing to their importance in CVD procedures [14], which is a developing industry for the production of thin films of the corresponding semiconducting materials [15]. As far as the authors are aware, only a handful of group 13 antimonides that contain AldSb σ-bonds have been discovered [16], No triply bonded RAl☰SbR species, which is isoelectronic to HC☰CH, has been reported both experimentally and theoretically.

Density functional theory (DFT) is sued to determine the structures, the kinetic stability and bonding properties of various RAl☰SbR triply bonded forms on the singlet ground state, in order to obtain a better understanding of aluminum☰antimony triple bonds. This work reports the possible existence of triply bonded RAl☰SbR molecules, from the viewpoint of the effect of substituents, using DFT [17]. That is, M06-2X/Def2-TZVP, B3PW91/Def2-TZVP

Scheme 1. Four bulky ligands, which are SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*.

and B3LYP/LANL2DZ + dp are used for small substituents (R = H, F, OH, CH3, and SiH3) and M06-2X/Def2-TZVP [18] for large substituents (R = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*; see Scheme 1) [19].

### 2. General considerations

group 14 atoms has increased during the last two decades, the understanding of the RE13☰E15R compounds, which are isoelectronic to acetylene from a valence electron viewpoint, is still limited. The reason for this limited knowledge of acetylene analogues, RE13☰E15R, could be due to the fact that there has been limited preparation and the isolation of these species in a stable form [11, 12]. Theoretical methods allow a theoretical design of the RE13☰E15R molecules to be made that increases understanding of their potential properties. The III-V semiconductors that contain antimony have several important applications in optoelectronic devices that operate in the infrared region and in high-speed devices, which has prompted widespread studies of promising precursor systems for these materials [13]. In particular, the chemical synthesis and structural characterization of AlSb single-source precursors of the type R3Al-SbR´3 has attracted much attention, owing to their importance in CVD procedures [14], which is a developing industry for the production of thin films of the corresponding semiconducting materials [15]. As far as the authors are aware, only a handful of group 13 antimonides that contain AldSb σ-bonds have been discovered [16], No triply bonded RAl☰SbR species, which is isoelectronic to HC☰CH, has been reported both experi-

84 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

Density functional theory (DFT) is sued to determine the structures, the kinetic stability and bonding properties of various RAl☰SbR triply bonded forms on the singlet ground state, in order to obtain a better understanding of aluminum☰antimony triple bonds. This work reports the possible existence of triply bonded RAl☰SbR molecules, from the viewpoint of the effect of substituents, using DFT [17]. That is, M06-2X/Def2-TZVP, B3PW91/Def2-TZVP

Scheme 1. Four bulky ligands, which are SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*.

mentally and theoretically.

The valence-bond bonding model is a well-known satisfactory method, which is an approximate theory to explain the electron pair or chemical bond by quantum mechanics, for predicting molecular geometries [20]. Two valence-bond bonding models (Figure 1) are thus used to interpret the bonding properties of triply bonded RAl☰SbR species. In model [1], the RAl☰SbR molecule is partitioned into two units: a singlet RdAl and a singlet RdSb. In model [2], the RAl☰SbR compound is divided into two moieties: a triplet RdAl and a triplet RdSb.

Figure 1. The valence-bond bonding models [1, 2] for the triply bonded RAl☰SbR molecule.

As a result, the choice of the bonding model that is used to explain the bonding characters of RAl☰SbR depends on the promotion energies (ΔEST = Etriplet Esinglet) of the RdAl and RdSb fragments. According to current theoretical calculations (see below), it is known that RdAl occupies the singlet ground state, but RdSb occupies the triplet ground state. In consequence, if the value of ΔEST for RdAl is much larger than that for RdSb, the latter easily jumps to the singlet excited state. Hence, model [1] can be used to explain the bonding nature of the RAl☰SbR molecule. In contrast, if the value of ΔEST for RdAl is smaller than that for RdSb, the former is readily promoted to the excited triplet state. Therefore, model [2] is used to interpret the bond constitutions of the RAl☰SbR compound.

Two points are worthy of note. The first is that since aluminum and antimony respectively belong to group 13 and group 15 and both elements have different atomic radii (covalent radii: 118 pm and 140 for Al and Sb, respectively) [20], the overlapping populations between Al and Sb should not be strong. The second is that the lone pairs of both aluminum and antimony feature the valence s character. This, in turn, makes the overlap integrals between the lone pair orbital and the pure p orbital small. These two factors mean that the triple bond between aluminum and antimony is weak, unlike the traditional triple bond in acetylene.

Bearing the above bonding analyses in mind, theoretical evidences are given in the following sections.

### 3. Results and discussion

### 3.1. Small ligands on substituted RAl☰SbR

Five small substituents (R = F, OH, H, CH3 and SiH3) are chosen, which include electronegative and electropositive groups, to determine their stability and bonding properties on the triply bonded RAl☰SbR molecules using the three types of DFT calculations (i.e., M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ + dp). Figure 2 shows the potential energy surfaces of the intra-molecular 1,2-migration reactions for five triply bonded RAl☰SbR compounds that feature small substituents. That is to say, the triply bonded RAl☰SbR species can undergo a 1,2-shift to give either R2Al〓Sb: or: Al〓SbR2 doubly bonded isomers.

As seen in Figure 2, the three DFT computational results demonstrate that the triply bonded RAl☰SbR species that feature small substituents are all both kinetically and thermodynamically unstable on the intra-molecular 1,2-migration reaction potential energy surfaces. In other words, once the triply bonded RAl☰SbR with small substituents is formed, it can easily proceed along the 1,2-migration to give the thermodynamically stable doubly bonded isomer, either R2Al〓Sb: or: Al〓SbR2. The theoretical findings give strong evidence that the triply bonded RAl☰SbR molecules that feature the small ligands are highly unlikely to be detected experimentally.

Although current theoretical observations show that the formation of RAl☰SbR involving small ligands is not likely, some of their physical properties, which are shown in Table 1, must be theoretically determined in order to design much more stable aluminum☰antimony acetylene analogues.

Figure 2. The 1,2-migration energy surfaces for RAl☰SbR (R = H, F, CH3, OH, and SiH3). These relative Gibbs free energies (kcal/mol) are computed at the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ + dp levels of theory.

The Triply Bonded Al≡Sb Molecules: A Theoretical Prediction

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87

As a result, the choice of the bonding model that is used to explain the bonding characters of RAl☰SbR depends on the promotion energies (ΔEST = Etriplet Esinglet) of the RdAl and RdSb fragments. According to current theoretical calculations (see below), it is known that RdAl occupies the singlet ground state, but RdSb occupies the triplet ground state. In consequence, if the value of ΔEST for RdAl is much larger than that for RdSb, the latter easily jumps to the singlet excited state. Hence, model [1] can be used to explain the bonding nature of the RAl☰SbR molecule. In contrast, if the value of ΔEST for RdAl is smaller than that for RdSb, the former is readily promoted to the excited triplet state. Therefore, model [2] is used to

Two points are worthy of note. The first is that since aluminum and antimony respectively belong to group 13 and group 15 and both elements have different atomic radii (covalent radii: 118 pm and 140 for Al and Sb, respectively) [20], the overlapping populations between Al and Sb should not be strong. The second is that the lone pairs of both aluminum and antimony feature the valence s character. This, in turn, makes the overlap integrals between the lone pair orbital and the pure p orbital small. These two factors mean that the triple bond between

Bearing the above bonding analyses in mind, theoretical evidences are given in the following

Five small substituents (R = F, OH, H, CH3 and SiH3) are chosen, which include electronegative and electropositive groups, to determine their stability and bonding properties on the triply bonded RAl☰SbR molecules using the three types of DFT calculations (i.e., M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ + dp). Figure 2 shows the potential energy surfaces of the intra-molecular 1,2-migration reactions for five triply bonded RAl☰SbR compounds that feature small substituents. That is to say, the triply bonded RAl☰SbR species can undergo a

As seen in Figure 2, the three DFT computational results demonstrate that the triply bonded RAl☰SbR species that feature small substituents are all both kinetically and thermodynamically unstable on the intra-molecular 1,2-migration reaction potential energy surfaces. In other words, once the triply bonded RAl☰SbR with small substituents is formed, it can easily proceed along the 1,2-migration to give the thermodynamically stable doubly bonded isomer, either R2Al〓Sb: or: Al〓SbR2. The theoretical findings give strong evidence that the triply bonded RAl☰SbR molecules that feature the small ligands are highly unlikely to be detected

Although current theoretical observations show that the formation of RAl☰SbR involving small ligands is not likely, some of their physical properties, which are shown in Table 1, must be theoretically determined in order to design much more stable aluminum☰antimony acety-

aluminum and antimony is weak, unlike the traditional triple bond in acetylene.

1,2-shift to give either R2Al〓Sb: or: Al〓SbR2 doubly bonded isomers.

interpret the bond constitutions of the RAl☰SbR compound.

86 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

sections.

experimentally.

lene analogues.

3. Results and discussion

3.1. Small ligands on substituted RAl☰SbR

Figure 2. The 1,2-migration energy surfaces for RAl☰SbR (R = H, F, CH3, OH, and SiH3). These relative Gibbs free energies (kcal/mol) are computed at the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ + dp levels of theory.


structures of RAl☰SbR adopt the bent form, as demonstrated in Scheme 2. That is, ∠RdAldSb ≈ 180.0 and ∠AldSbdR ≈ 90.0. The reason for this vertical angle at the Sb center can be ascribed to the relativistic effect, as discussed previously [21]. The three DFT calculations shown in Table 1 all indicate that the electronic ground states for RdAl and the RdSb fragments are singlet and triplet, respectively. In particular, all of the DFT results shown in Table 1 show that most of the singlet-triplet energy splitting (ΔEST) of R-Al is larger than that of the corresponding RdSb. This strongly implies that the bonding characters of the triply bonded RAl☰SbR species that feature small substituents are better described by model [1], as shown in Figure 1. In other words, the triple bond consists of one donor-acceptor σ bond and two donor-acceptor π bonds, which are schematically represented as RdAl SbdR. As previously mentioned, since the lone pair orbitals of both the R-Al and the R-Sb fragments feature the valence s character, their overlapping populations between the lone orbital and the valence p orbital should be smaller. Indeed, the supporting evidence from Table 1 shows that all bond orders for the RAl☰SbR species are estimated to be less than 2.0 (WBI = 1.474–1.799),

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89

Scheme 2. The geometrical structure of RAl☰SbR with the small substituent, R.

which is less than the bond order for the C☰C triple bond in acetylene (WBI = 2.99).

bond.

3.2. Large ligands on substituted R'Al☰SbR´

In brief, the three DFT calculations shown in this work show that irrespective of their electronegativity, the triply bonded RAl☰SbR molecules that feature small ligands are highly unlikely to exist, even in the low-temperature matrices. In particular, the bond orders of these Al☰Sb triple bonds are theoretically predicted to be a weak double bond, rather than a triple

Three bulky groups were then used to search for kinetically stable triple-bonded R'Al☰SbR´ molecules: R´(〓SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*) [19]. These are shown in Scheme 1. It is known that London dispersion (nonvalent interactions) plays a prominent role in both chemical and physical properties of inorganic molecules [23]. As a result, the dispersion-corrected M06-2X/Def2-TZVP method is used in the present study to investigate the behaviors of the triply bonded R'Al☰SbR´ compounds bearing bulky substituents. Similarly to the cases for small ligands on substituted RAl☰SbR, the dispersion-corrected M06-2X/Def2-TZVP level of theory is used to determine the potential energy surfaces for the intra-molecular 1,2-migration

1 The charge density on the Al element.

2 The charge density on the Sb element.

3 <sup>Δ</sup>EST = E(triplet state for RdAl) E(singlet state for RdSb). <sup>4</sup>

<sup>Δ</sup>EST = E(triplet state for RdAl) E(singlet state for RdSb). <sup>5</sup>

BE = E(singlet state for RdAl) + E(triplet state for RdSb) E(singlet state for RAl☰SbR). <sup>6</sup>

The Wiberg bond index (WBI) for the Al☰Sb bond: see Ref. [22].

Table 1. The key geometrical parameters, the singlet-triplet energy splitting (ΔEST), the natural charge densities (QAl and QSb), the binding energies (BE), the HOMO-LUMO energy gaps, and the Wiberg bond index (WBI) for RAl☰SbR using the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP (in round brackets) and B3LYP/LANL2DZ + dp (in square brackets) levels of theory.

As seen in Table 1, the three DFT computational results predict that the Al☰Sb triple bond distance (Å) is in the ranges 2.388–2.539 (M06-2X/Def2-TZVP), 2.397–2.536 (B3PW91/Def2- TZVP) and 2.436–2.565 (B3LYP/LANL2DZ + dp). Table 1 also shows that all of the geometrical

Scheme 2. The geometrical structure of RAl☰SbR with the small substituent, R.

structures of RAl☰SbR adopt the bent form, as demonstrated in Scheme 2. That is, ∠RdAldSb ≈ 180.0 and ∠AldSbdR ≈ 90.0. The reason for this vertical angle at the Sb center can be ascribed to the relativistic effect, as discussed previously [21]. The three DFT calculations shown in Table 1 all indicate that the electronic ground states for RdAl and the RdSb fragments are singlet and triplet, respectively. In particular, all of the DFT results shown in Table 1 show that most of the singlet-triplet energy splitting (ΔEST) of R-Al is larger than that of the corresponding RdSb. This strongly implies that the bonding characters of the triply bonded RAl☰SbR species that feature small substituents are better described by model [1], as shown in Figure 1. In other words, the triple bond consists of one donor-acceptor σ bond and two donor-acceptor π bonds, which are schematically represented as RdAl SbdR. As previously mentioned, since the lone pair orbitals of both the R-Al and the R-Sb fragments feature the valence s character, their overlapping populations between the lone orbital and the valence p orbital should be smaller. Indeed, the supporting evidence from Table 1 shows that all bond orders for the RAl☰SbR species are estimated to be less than 2.0 (WBI = 1.474–1.799), which is less than the bond order for the C☰C triple bond in acetylene (WBI = 2.99).

In brief, the three DFT calculations shown in this work show that irrespective of their electronegativity, the triply bonded RAl☰SbR molecules that feature small ligands are highly unlikely to exist, even in the low-temperature matrices. In particular, the bond orders of these Al☰Sb triple bonds are theoretically predicted to be a weak double bond, rather than a triple bond.

### 3.2. Large ligands on substituted R'Al☰SbR´

As seen in Table 1, the three DFT computational results predict that the Al☰Sb triple bond distance (Å) is in the ranges 2.388–2.539 (M06-2X/Def2-TZVP), 2.397–2.536 (B3PW91/Def2- TZVP) and 2.436–2.565 (B3LYP/LANL2DZ + dp). Table 1 also shows that all of the geometrical

Table 1. The key geometrical parameters, the singlet-triplet energy splitting (ΔEST), the natural charge densities (QAl and QSb), the binding energies (BE), the HOMO-LUMO energy gaps, and the Wiberg bond index (WBI) for RAl☰SbR using the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP (in round brackets) and B3LYP/LANL2DZ + dp (in square brackets) levels

R F OH H CH3 SiH3

2.531 (2.518) [2.565]

173.4 (172.0) [176.5]

86.55 (86.13) [90.43]

179.7 (176.9) [178.6]

0.418 (0.401) [0.469]

0.196 (0.136) [0.119]

72.05 (65.86) [67.75]

25.88 (21.16) [20.04]

159.8 (140.1) [145.2]

22.77 (27.32) [21.96]

1.474 (1.550) [1.555] 2.388 (2.397) [2.436]

170.7 (167.6) [167.6]

82.25 (84.42) [86.43]

180.0 (180.0) [180.0]

0.164 (0.161) [0.414]

0.134 (0.107) [-0.032]

43.73 (40.25) [40.80]

33.35 (29.42) [27.91]

257.6 (205.2) [277.6]

55.28 (64.05) [56.79]

1.754 (1.799) [1.779] 2.466 (2.462) [2.499]

177.7 (173.8) [173.2]

94.46 (96.42) [96.75]

179.6 (179.9) [178.2]

0.291 (0.262) [0.282]

0.054 (0.018) [0.134]

48.75 (42.38) [45.00]

31.52 (27.31) [26.00]

146.4 (123.3) [129.2]

42.23 (51.72) [46.41]

1.659 (1.714) [1.733] 2.539 (2.524) [2.560]

176.8 (176.2) [179.7]

88.86 (88.07) [88.53]

179.9 (179.9) [180.0]

0.208 (0.219) [0.193]

0.198 (0.100) [0.179]

32.87 (29.08) [31.97]

30.78 (25.61) [25.21]

172.2 (179.5) [177.9]

61.00 (67.80) [57.43]

1.581 (1.596) [1.637]

AlαSb (Å) 2.528

∠R-Al-Sb () 176.8

∠Al-Sb-R () 88.86

∠R-Sb-Al-R () 179.9

QAl1 0.5201

QSb<sup>2</sup> 0.329

ΔEST for Al-R (kcal/mol)3 79.78

<sup>Δ</sup>EST for Sb-R (kcal/mol)<sup>4</sup> 32.40

HOMO-LUMO (kcal/mol) 165.5

BE (kcal/mol)<sup>5</sup> 25.82

WBI6 1.483

The charge density on the Al element.

The charge density on the Sb element.

<sup>Δ</sup>EST = E(triplet state for RdAl) E(singlet state for RdSb). <sup>4</sup> <sup>Δ</sup>EST = E(triplet state for RdAl) E(singlet state for RdSb). <sup>5</sup>

The Wiberg bond index (WBI) for the Al☰Sb bond: see Ref. [22].

1

2

3

of theory.

(2.536) [2.556]

88 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

(176.2) [179.2]

(88.07) [88.53]

(179.9) [180.0]

(0.495) [0.715]

(0.277) [0.217]

(71.44) [73.78]

(28.88) [27.52]

(168.4) [167.2]

(32.05) [27.43]

(1.556) [1.560]

BE = E(singlet state for RdAl) + E(triplet state for RdSb) E(singlet state for RAl☰SbR). <sup>6</sup>

Three bulky groups were then used to search for kinetically stable triple-bonded R'Al☰SbR´ molecules: R´(〓SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*) [19]. These are shown in Scheme 1. It is known that London dispersion (nonvalent interactions) plays a prominent role in both chemical and physical properties of inorganic molecules [23]. As a result, the dispersion-corrected M06-2X/Def2-TZVP method is used in the present study to investigate the behaviors of the triply bonded R'Al☰SbR´ compounds bearing bulky substituents. Similarly to the cases for small ligands on substituted RAl☰SbR, the dispersion-corrected M06-2X/Def2-TZVP level of theory is used to determine the potential energy surfaces for the intra-molecular 1,2-migration reactions of R'Al☰SbR´, as shown in Scheme 3. The computed relative energies are listed in Table 2. The reaction enthalpies for both the 1,2-shift reactions (R'Al☰SbR´ ! R2'Al〓Sb and R'Al☰SbR´ ! R2'Sb〓Al) are apparently too high. They are estimated to be at least 80 kcal/mol. The reason that both doubly bonded R2'Al〓Sb and R2'Sb〓Al isomers occupy such high energy points is simply because two bulky groups can cause steric overcrowding. As a consequence, the theoretical findings strongly suggest that the triply bonded R'Al☰SbR´, which is attached by two bulkier substituents, is kinetically stabilized.

Table 2 shows that the Al☰Sb triple bond distance is predicted to be 2.422–2.477 Å. Since no experimental results for the Al☰Sb triple bond length have been reported, these values are estimates. These theoretical calculations also show that the geometrical structures of R0 Al☰SbR<sup>0</sup> molecules that feature bulky groups adopt a bent structure; i.e., ∠R<sup>0</sup> dAldSb ≈ 160.0� and ∠AldSbdR<sup>0</sup> ≈ 120.0�. As stated previously, the triply bonded R<sup>0</sup> Al☰SbR<sup>0</sup> species feature this bent geometry because of the relativistic effect [23].

In addition, the bonding energy (BE) that is shown in Table 2 shows that the central aluminum and antimony atoms in the substituted R<sup>0</sup> Al☰SbR<sup>0</sup> compounds are strongly bonded, since the

BE values are in the range 71–97 kcal/mol for R<sup>0</sup> = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*. Table 2 also shows that the modulus ΔEST (kcal/mol) for AldR<sup>0</sup> and SbdR<sup>0</sup> fragments are predicted to be 43–27 and 31–16. These theoretical values allow two interpretations. Firstly, even when attached by bulkier groups, it is theoretically verified that both the AldR<sup>0</sup> and the SbdR<sup>0</sup> units occupy the ground singlet state and the ground triplet state, respectively. Since the ΔEST values for AldR<sup>0</sup> are so small (compared with those for AldR, as shown in Table 1), model [2] in Figure 1 is most suitable to interpret the triple bonding characters in the

Table 2. The key geometrical parameters, the singlet-triplet energy splitting (ΔEST), the natural charge densities (QAl and QSb), the binding energies (BE), the HOMO-LUMO energy gaps, reaction enthalpies, and the Wiberg bond index (WBI)

dAl) � E(singlet state for R<sup>0</sup>

dSb) � E(singlet state for R<sup>0</sup>

dAl) + E(triplet state for R<sup>0</sup>

R<sup>0</sup> SiMe(SitBu3)2 SiiPrDis2 Tbt Ar\* Al☰Sb (Å) 2.463 2.422 2.477 2.447

dAldSb (�) 157.6 152.0 161.3 165.0

QAl1 0.619 0.637 1.008 1.027 QSb<sup>2</sup> �0.387 �0.492 �0.025 �0.114 ΔEST for AldR<sup>0</sup> (kcal/mol)3 28.89 27.30 42.50 40.21 <sup>Δ</sup>EST for SbdR<sup>0</sup> (kcal/mol)<sup>4</sup> �16.89 �24.80 �30.51 �15.92 HOMO-LUMO (kcal/mol) 53.56 60.07 56.08 56.68 BE (kcal/mol)<sup>5</sup> 71.29 72.97 87.43 74.33 ΔH1 (kcal/mol)6 94.23 84.67 92.12 82.68 ΔH2 (kcal/mol)6 83.15 84.08 80.01 88.19 WBI7 2.174 2.181 2.072 2.016

�) 126.5 123.6 122.2 124.6

�) 173.5 172.9 167.2 166.0

dAl).

dSb).

dSb) � E(singlet for R<sup>0</sup>

Al☰SbR<sup>0</sup> ).

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91

Al☰SbR<sup>0</sup> species that feature bulky substituents. As schematically shown in Figure 1, the nature of the Al☰Sb triple bond can be considered as one conventional σ bond, one conven-

that two factors affect the overlapping populations between the central Al and Sb elements. The first is that the lone pair orbital of the SbdR<sup>0</sup> moiety features the valence s character. This, in turn, renders the overlap population between the pure p orbital of Al and the lone pair orbital of Sb very small. The other is that the sizes of the valence p orbitals for Al and Sb are quite different, since they belong to different rows of the periodic table having different

dAl SbdR<sup>0</sup>

. It is worthy of note

Al☰SbR<sup>0</sup> molecules that feature

tional π bond and one donor-acceptor π bond. That is, R<sup>0</sup>

principal quantum numbers. As a result, the triple bond in R<sup>0</sup>

R0

∠R<sup>0</sup>

1

2

3

4

5

6

7

for R<sup>0</sup>

ΔEST (kcal mol�<sup>1</sup>

ΔEST (kcal mol�<sup>1</sup>

See also Scheme 3.

BE (kcal mol�<sup>1</sup>

See Scheme 3.

The charge density on the Al element.

The charge density on the Sb element.

) = E(triplet state for R<sup>0</sup>

) = E(triplet state for R<sup>0</sup>

The Wiberg bond index (WBI) for the AlαSb bond: see Ref. [22].

Al☰SbR<sup>0</sup> at the dispersion-corrected M06-2X/Def2-TZVP level of theory.

) = E(triplet state for R<sup>0</sup>

∠AldSbdR<sup>0</sup> (

∠R'dAldSbdR<sup>0</sup> (

Scheme 3. The qualitative potential energy surface of the R´Al☰SbR´ isomers with the bulky substituent, R´.


1 The charge density on the Al element.

2 The charge density on the Sb element.

3 ΔEST (kcal mol�<sup>1</sup> ) = E(triplet state for R<sup>0</sup> dAl) � E(singlet state for R<sup>0</sup> dAl).

4 ΔEST (kcal mol�<sup>1</sup> ) = E(triplet state for R<sup>0</sup> dSb) � E(singlet state for R<sup>0</sup> dSb).

5 BE (kcal mol�<sup>1</sup> ) = E(triplet state for R<sup>0</sup> dAl) + E(triplet state for R<sup>0</sup> dSb) � E(singlet for R<sup>0</sup> Al☰SbR<sup>0</sup> ).

6 See Scheme 3.

reactions of R'Al☰SbR´, as shown in Scheme 3. The computed relative energies are listed in Table 2. The reaction enthalpies for both the 1,2-shift reactions (R'Al☰SbR´ ! R2'Al〓Sb and R'Al☰SbR´ ! R2'Sb〓Al) are apparently too high. They are estimated to be at least 80 kcal/mol. The reason that both doubly bonded R2'Al〓Sb and R2'Sb〓Al isomers occupy such high energy points is simply because two bulky groups can cause steric overcrowding. As a consequence, the theoretical findings strongly suggest that the triply bonded R'Al☰SbR´, which is

Table 2 shows that the Al☰Sb triple bond distance is predicted to be 2.422–2.477 Å. Since no experimental results for the Al☰Sb triple bond length have been reported, these values are estimates. These theoretical calculations also show that the geometrical structures of

In addition, the bonding energy (BE) that is shown in Table 2 shows that the central aluminum

dAldSb

Al☰SbR<sup>0</sup> species

Al☰SbR<sup>0</sup> compounds are strongly bonded, since the

Al☰SbR<sup>0</sup> molecules that feature bulky groups adopt a bent structure; i.e., ∠R<sup>0</sup>

Scheme 3. The qualitative potential energy surface of the R´Al☰SbR´ isomers with the bulky substituent, R´.

≈ 160.0� and ∠AldSbdR<sup>0</sup> ≈ 120.0�. As stated previously, the triply bonded R<sup>0</sup>

attached by two bulkier substituents, is kinetically stabilized.

90 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

feature this bent geometry because of the relativistic effect [23].

and antimony atoms in the substituted R<sup>0</sup>

R0

7 The Wiberg bond index (WBI) for the AlαSb bond: see Ref. [22].

See also Scheme 3.

Table 2. The key geometrical parameters, the singlet-triplet energy splitting (ΔEST), the natural charge densities (QAl and QSb), the binding energies (BE), the HOMO-LUMO energy gaps, reaction enthalpies, and the Wiberg bond index (WBI) for R<sup>0</sup> Al☰SbR<sup>0</sup> at the dispersion-corrected M06-2X/Def2-TZVP level of theory.

BE values are in the range 71–97 kcal/mol for R<sup>0</sup> = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*. Table 2 also shows that the modulus ΔEST (kcal/mol) for AldR<sup>0</sup> and SbdR<sup>0</sup> fragments are predicted to be 43–27 and 31–16. These theoretical values allow two interpretations. Firstly, even when attached by bulkier groups, it is theoretically verified that both the AldR<sup>0</sup> and the SbdR<sup>0</sup> units occupy the ground singlet state and the ground triplet state, respectively. Since the ΔEST values for AldR<sup>0</sup> are so small (compared with those for AldR, as shown in Table 1), model [2] in Figure 1 is most suitable to interpret the triple bonding characters in the R0 Al☰SbR<sup>0</sup> species that feature bulky substituents. As schematically shown in Figure 1, the nature of the Al☰Sb triple bond can be considered as one conventional σ bond, one conventional π bond and one donor-acceptor π bond. That is, R<sup>0</sup> dAl SbdR<sup>0</sup> . It is worthy of note that two factors affect the overlapping populations between the central Al and Sb elements. The first is that the lone pair orbital of the SbdR<sup>0</sup> moiety features the valence s character. This, in turn, renders the overlap population between the pure p orbital of Al and the lone pair orbital of Sb very small. The other is that the sizes of the valence p orbitals for Al and Sb are quite different, since they belong to different rows of the periodic table having different principal quantum numbers. As a result, the triple bond in R<sup>0</sup> Al☰SbR<sup>0</sup> molecules that feature bulky substituents is predicted to be quite weak. Indeed, the theoretical evidences given in Table 2 shows that the bond order is a little bit higher than 2.0 (WBI ≈ 2.17, 2.18, 2.07 and 2.02 for R<sup>0</sup> = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*, respectively). The bond order for the conventional C☰C bond in acetylene is estimated to be 2.99.

The bonding characters of the Al☰Sb triple bond in R<sup>0</sup>

R' = SiMe (SitBu3)2

R0

Def2-TZVP level of theory.

using the natural bond orbital (NBO) [22] and the natural resonance theory (NRT) [25] analysis, whose results are given in Table 4, are used to determine the bonding properties. For instance, Table 4 shows that for (SiMe(SitBu3)2)Al☰Sb(SiMe(SitBu3)2), the NBO model shows that the Al-Sb σ bonding orbital contains about 23% natural Al orbitals and 77% natural Sb orbitals. Also, the Al☰Sb π bonding orbital contains averagely about 25% natural Al orbitals and 75% natural Sb orbitals (Figure 3). These values give strong evidence that the Al☰Sb π bond is polarized. Table 4 also shows that the Al☰Sb π bonding interaction: π⊥(Al☰Sb) = 0.529 (3s3p1.98)Al + 0.849(5s5p12.43)Sb and <sup>π</sup>k(Al☰Sb) = 0.475(3s3p99.99)Al + 0.880(5s5p99.99)Sb, which again implies that the predominant bonding interaction between the AldR and the SbdR moieties originates from 3p(Al) 5p(Sb) donation. In other words, the electron deficiency on Al and the π bond polarity are partially balanced by the donation of the Sb lone pair to the empty Al p orbital (Figure 3). Table 4 also shows that, on the basis of the NRT analyses of the electron density for (SiMe(SitBu3)2)Al☰Sb(SiMe(SitBu3)2), its Al☰Sb triple bond has a greater

R'Al☰SbR' WBI NBO analysis NRT analysis

2.17 σ: 1.91 σ: 0.4799 Al (sp3.23) + 0.8773 Sb (sp0.60)

(sp12.43)

(sp99.99)

(sp1.15)

(sp3.68)

(sp99.99)

(sp12.38)

(sp99.99)

(sp99.99)

(sp18.14)

(sp40.30)

(sp99.99)

R' = SiiPrDis2 2.18 σ: 1.91 σ: 0.5525 Al (sp1.71) + 0.8335 Sb

R' = Tbt 2.07 σ: 1.95 σ: 0.6923 Al (sp0.18) + 0.7216 Sb

R' = Ar\* 2.02 σ: 1.96 σ: 0.6946 Al (sp0.16) + 0.7194 Sb

<sup>π</sup>k: 1.89 <sup>π</sup>k: 0.4753 Al (sp99.99) + 0.8798 Sb

<sup>π</sup>k: 1.89 <sup>π</sup>k: 0.4476 Al (sp99.99) + 0.8943 Sb

<sup>π</sup>k: 1.91 <sup>π</sup>k: 0.4772 Al (sp99.99) + 0.8788 Sb

<sup>π</sup>k: 1.92 <sup>π</sup>k: 0.4266 Al (sp99.99) + 0.9044 Sb

Occupancy Hybridization Polarization Total/covalent/

Al☰SbR<sup>0</sup> molecules were examined

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The Triply Bonded Al≡Sb Molecules: A Theoretical Prediction

ionic

23.03% (Al) 76.97% (Sb)

27.96% (Al) 72.04% (Sb)

22.59% (Al) 77.41% (Sb)

30.53% (Al) 69.47% (Sb)

22.30% (Al) 77.70% (Sb)

20.03% (Al) 79.97% (Sb)

47.93% (Al) 52.07% (Sb)

20.14% (Al) 79.86% (Sb)

22.78% (Al) 77.22% (Sb)

48.25% (Al) 51.75% (Sb)

20.64% (Al) 79.36% (Sb)

18.20% (Al) 81.80% (Sb)

Al☰Sb: 17.21% <sup>π</sup>⊥: 1.81 <sup>π</sup>⊥: 0.5288 Al (sp1.98) + 0.8487 Sb

Al☰Sb: 13.84% <sup>π</sup>⊥: 1.86 <sup>π</sup>⊥: 0.4723 Al (sp3.67) + 0.8815 Sb

Al☰Sb: 28.22% <sup>π</sup>⊥: 1.88 <sup>π</sup>⊥: 0.4488 Al (sp47.14) + 0.8936 Sb

Al☰Sb: 11.87% <sup>π</sup>⊥: 1.83 <sup>π</sup>⊥: 0.4543 Al (sp99.99) + 0.8908 Sb

Table 4. The natural bond orbital (NBO), the natural resonance theory (NRT) analysis, and Wiberg bond index (WBI) for

Al☰SbR<sup>0</sup> molecules that feature ligands (R<sup>0</sup> = SiMe(SitBu3)2, SiiPrDis2, and NHC) at the dispersion-corrected M06-2X/

Resonance weight

Al〓Sb: 71.95%

Al〓Sb: 75.53%

Al〓Sb: 65.89%

Al〓Sb: 76.76%

2.06/1.25/0.81 AldSb: 10.84%

2.48/1.29/1.19 AldSb: 10.63%

2.22/1.41/0.82 AldSb: 5.89%

2.01/1.44/0.57 AldSb: 11.37%

Besides these, Dapprich and Frenking developed a useful method [24], which is called the introduced charge decomposition analysis (CDA), from which one may analyze donoracceptor interactions of a A-B molecule. From CDA, one may obtain three parts. The first part is the number of electrons donated from the R<sup>0</sup> dAl unit to the R<sup>0</sup> dSb monomer, which can be considered as (R<sup>0</sup> dAl) ! (R<sup>0</sup> dSb). The second part is the number of electrons back donated from the R<sup>0</sup> dSb component to the R<sup>0</sup> dAl moiety, which can be represented as (R<sup>0</sup> dAl) (R<sup>0</sup> dSb). The third part is the repulsive interactions between (R<sup>0</sup> dAl) and (R<sup>0</sup> dSb), which can be described as (R<sup>0</sup> dAl) \$ (R<sup>0</sup> dSb). The CDA results about the (SiMe(SitBu3)2)Al☰Sb(SiMe (SitBu3)2) molecule based on the dispersion-corrected M06-2X/Def2-TZVP method are given in Table 3. As seen in Table 3, for the (R<sup>0</sup> dSb) fragment, its largest contribution is No. 267 (HOMO) orbital, displaying that a R<sup>0</sup> dSb component donates electrons to a R<sup>0</sup> dGa unit mainly through the HOMO orbital. In consequence, the net amount of electron transfer is estimated to be �0.207, implying that the R<sup>0</sup> dSb part donates more electrons to the R<sup>0</sup> dAl moiety. This theoretical finding agrees well with the valence-electron bonding model shown in Figure 1 (i.e., model [2]). Namely, the bonding character of R<sup>0</sup> Al☰SbR<sup>0</sup> can be recognized as R0 Al SbR<sup>0</sup> .


For clearness, only list the X, Y, and W terms for HOMO(no.267)-11 � LUMO+2. <sup>a</sup> Summation of contributions from all unoccupied and occupied orbitals.

Table 3. The charge decomposition analysis (CDA) for R<sup>0</sup> Al☰SbR<sup>0</sup> (R<sup>0</sup> = SiMe(SitBu3)2) system based on M06-2X orbitals, where a is the number of electrons donating from R<sup>0</sup> dAl unit to R<sup>0</sup> dSb unit, B is the number of electrons donating from R0 -Sb moiety to R<sup>0</sup> -Al moiety and W is the number of electrons involved in repulsive polarization.

The bonding characters of the Al☰Sb triple bond in R<sup>0</sup> Al☰SbR<sup>0</sup> molecules were examined using the natural bond orbital (NBO) [22] and the natural resonance theory (NRT) [25] analysis, whose results are given in Table 4, are used to determine the bonding properties. For instance, Table 4 shows that for (SiMe(SitBu3)2)Al☰Sb(SiMe(SitBu3)2), the NBO model shows that the Al-Sb σ bonding orbital contains about 23% natural Al orbitals and 77% natural Sb orbitals. Also, the Al☰Sb π bonding orbital contains averagely about 25% natural Al orbitals and 75% natural Sb orbitals (Figure 3). These values give strong evidence that the Al☰Sb π bond is polarized. Table 4 also shows that the Al☰Sb π bonding interaction: π⊥(Al☰Sb) = 0.529 (3s3p1.98)Al + 0.849(5s5p12.43)Sb and <sup>π</sup>k(Al☰Sb) = 0.475(3s3p99.99)Al + 0.880(5s5p99.99)Sb, which again implies that the predominant bonding interaction between the AldR and the SbdR moieties originates from 3p(Al) 5p(Sb) donation. In other words, the electron deficiency on Al and the π bond polarity are partially balanced by the donation of the Sb lone pair to the empty Al p orbital (Figure 3). Table 4 also shows that, on the basis of the NRT analyses of the electron density for (SiMe(SitBu3)2)Al☰Sb(SiMe(SitBu3)2), its Al☰Sb triple bond has a greater

bulky substituents is predicted to be quite weak. Indeed, the theoretical evidences given in Table 2 shows that the bond order is a little bit higher than 2.0 (WBI ≈ 2.17, 2.18, 2.07 and 2.02 for R<sup>0</sup> = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar\*, respectively). The bond order for the conven-

Besides these, Dapprich and Frenking developed a useful method [24], which is called the introduced charge decomposition analysis (CDA), from which one may analyze donoracceptor interactions of a A-B molecule. From CDA, one may obtain three parts. The first part

(SitBu3)2) molecule based on the dispersion-corrected M06-2X/Def2-TZVP method are given in

mainly through the HOMO orbital. In consequence, the net amount of electron transfer is

moiety. This theoretical finding agrees well with the valence-electron bonding model shown in

HOMO 267 2.000000 �0.000521 0.063131 �0.032203 �0.047961 LUMO 268 0.000000 0.000000 0.000000 0.000000 0.000000

Sum<sup>a</sup> 534.000000 0.043071 0.250110 �0.207039 �0.099090

dAl unit to R<sup>0</sup>


269 0.000000 0.000000 0.000000 0.000000 0.000000

Orbital Occupancy A B A-B W 2.000000 0.000897 0.000398 0.000499 0.000052 2.000000 �0.000691 �0.000223 �0.000469 �0.003158 2.000000 0.000003 0.000212 �0.000209 �0.000135 2.000000 �0.000574 0.001495 �0.002069 �0.003430 2.000000 0.000322 0.000997 �0.000676 �0.003797 2.000000 0.000333 0.000068 �0.002466 �0.012549 2.000000 0.000927 0.007097 0.000859 0.000836 2.000000 0.001417 0.031682 �0.003680 �0.003811 2.000000 0.005618 0.033540 �0.057513 �0.129159 2.000000 0.016174 0.031540 �0.017366 0.011841

dAl unit to the R<sup>0</sup>

dSb). The second part is the number of electrons back donated

dAl moiety, which can be represented as (R<sup>0</sup>

dSb). The CDA results about the (SiMe(SitBu3)2)Al☰Sb(SiMe

dSb component donates electrons to a R<sup>0</sup>

dAl) and (R<sup>0</sup>

dSb) fragment, its largest contribution is No. 267

dSb part donates more electrons to the R<sup>0</sup>

Al☰SbR<sup>0</sup> (R<sup>0</sup> = SiMe(SitBu3)2) system based on M06-2X orbitals,

dSb unit, B is the number of electrons donating from

dSb monomer, which can be

Al☰SbR<sup>0</sup> can be recognized as

dAl)

dGa unit

dAl

dSb), which can

tional C☰C bond in acetylene is estimated to be 2.99.

92 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

is the number of electrons donated from the R<sup>0</sup>

dAl) ! (R<sup>0</sup>

Table 3. As seen in Table 3, for the (R<sup>0</sup>

estimated to be �0.207, implying that the R<sup>0</sup>

(HOMO) orbital, displaying that a R<sup>0</sup>

dSb component to the R<sup>0</sup>

dAl) \$ (R<sup>0</sup>

dSb). The third part is the repulsive interactions between (R<sup>0</sup>

Figure 1 (i.e., model [2]). Namely, the bonding character of R<sup>0</sup>

For clearness, only list the X, Y, and W terms for HOMO(no.267)-11 � LUMO+2. <sup>a</sup> Summation of contributions from all unoccupied and occupied orbitals.

Table 3. The charge decomposition analysis (CDA) for R<sup>0</sup>

where a is the number of electrons donating from R<sup>0</sup>

considered as (R<sup>0</sup>

be described as (R<sup>0</sup>

from the R<sup>0</sup>

(R<sup>0</sup>

R0

R0


Al SbR<sup>0</sup>

.


Table 4. The natural bond orbital (NBO), the natural resonance theory (NRT) analysis, and Wiberg bond index (WBI) for R0 Al☰SbR<sup>0</sup> molecules that feature ligands (R<sup>0</sup> = SiMe(SitBu3)2, SiiPrDis2, and NHC) at the dispersion-corrected M06-2X/ Def2-TZVP level of theory.

bonded R<sup>0</sup>

represented as R<sup>0</sup>

structures of the R<sup>0</sup>

angle at the antimony center.

Acknowledgements

Author details

Kaohsiung, Taiwan

References

Jia-Syun Lu1

Taiwan for the financial support.

that the bonding characters of the R<sup>0</sup>

dAl SbdR<sup>0</sup>

Al☰SbR<sup>0</sup> compounds, and not small substituents. The theoretical findings also show

Al☰SbR<sup>0</sup> species adopt a bent conformation with a nearly perpendicular

. That is to say, the R<sup>0</sup>

σ bond, a conventional π bond and a donor-acceptor π bond. However, due to the poor overlapping populations between the Al and Sb elements, which is due to the different atomic sizes of the two elements and the nature of overlapping bonding orbitals, the Al☰Sb triple bond is very weak. The theoretical results also give strong evidence that the geometrical

The authors are grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time, and the Ministry of Science and Technology of

, Ming-Chung Yang1 and Ming-Der Su1,2\*

1 Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan

2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,

[1] Fischer RC, Power PP. π-Bonding and the lone pair effect in multiple bonds involving heavier main group elements: Developments in the new millennium. Chemical Reviews.

[2] Danovich D, Bino A, Shaik S. Formation of carbon–carbon triply bonded molecules from two free carbyne radicals via a conical intersection. Journal of Physical Chemistry Letters.

[3] Sasamori T, Hironaka K, Sugiyama T, Takagi N, Nagase S, Hosoi Y, Furukawa Y, Tokitoh N. Synthesis and reactions of a stable 1,2-diaryl-1,2-dibromodisilene: A precursor for substituted disilenes and a 1,2-diaryldisilyne. Journal of the American Chemical Society.

\*Address all correspondence to: midesu@mail.ncyu.edu.tw

2010;110:3877-3923. DOI: 10.1021/cr100133q

2008;130:13856-13857. DOI: 10.1021/ja8061002.

2013;4:58-64. DOI: 10.1021/jz3016765

Al☰SbR<sup>0</sup> species that feature bulky groups can be

The Triply Bonded Al≡Sb Molecules: A Theoretical Prediction

Al☰SbR<sup>0</sup> species contains a conventional

http://dx.doi.org/10.5772/intechopen.78412

95

Figure 3. The natural Al☰Sb π bonding orbitals ((i) and (ii)) for (SiMe(SitBu3)2)Al☰Sb(SiMe(SitBu3)2). Also, see Figure 1. (i) π⊥, (ii) πk.

covalent character, as shown by the greater covalent part of the NRT bond order (1.25), compared to its ionic part (0.81). The reason for this may be due to the fact that the difference between the electronegativity values for the Al and Sb elements is small (Al: 1.5 and Sb: 1.8) [26].

### 4. Conclusion

This study uses DFT computations to theoretically design substituted RAl☰SbR molecules that feature the Al☰Sb triple bond, that are stable from the kinetic viewpoint. The theoretical observations show that only bulky substituents (R<sup>0</sup> ) can significantly stabilize the triply bonded R<sup>0</sup> Al☰SbR<sup>0</sup> compounds, and not small substituents. The theoretical findings also show that the bonding characters of the R<sup>0</sup> Al☰SbR<sup>0</sup> species that feature bulky groups can be represented as R<sup>0</sup> dAl SbdR<sup>0</sup> . That is to say, the R<sup>0</sup> Al☰SbR<sup>0</sup> species contains a conventional σ bond, a conventional π bond and a donor-acceptor π bond. However, due to the poor overlapping populations between the Al and Sb elements, which is due to the different atomic sizes of the two elements and the nature of overlapping bonding orbitals, the Al☰Sb triple bond is very weak. The theoretical results also give strong evidence that the geometrical structures of the R<sup>0</sup> Al☰SbR<sup>0</sup> species adopt a bent conformation with a nearly perpendicular angle at the antimony center.

### Acknowledgements

The authors are grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time, and the Ministry of Science and Technology of Taiwan for the financial support.

### Author details

Jia-Syun Lu1 , Ming-Chung Yang1 and Ming-Der Su1,2\*

\*Address all correspondence to: midesu@mail.ncyu.edu.tw

1 Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan

2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan

### References

covalent character, as shown by the greater covalent part of the NRT bond order (1.25), compared to its ionic part (0.81). The reason for this may be due to the fact that the difference between the electronegativity values for the Al and Sb elements is small (Al: 1.5 and Sb: 1.8) [26].

Figure 3. The natural Al☰Sb π bonding orbitals ((i) and (ii)) for (SiMe(SitBu3)2)Al☰Sb(SiMe(SitBu3)2). Also, see Figure 1.

This study uses DFT computations to theoretically design substituted RAl☰SbR molecules that feature the Al☰Sb triple bond, that are stable from the kinetic viewpoint. The theoretical

) can significantly stabilize the triply

observations show that only bulky substituents (R<sup>0</sup>

94 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

4. Conclusion

(i) π⊥, (ii) πk.


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00002-7

CO;2-6


**Chapter 6**

**Provisional chapter**

**Periodic Trends among Interstellar Molecular Species:**

Out of the 19 known S-containing interstellar molecules, 16 have the corresponding O-compound analogues marks out of the interstellar chemistry of sulfur and oxygen as a unique one among other observed interstellar periodic trends. However, the rule that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio is far from reality in many cases even when both species are observed from the same source. In this chapter, the effect of interstellar hydrogen bonding on the variation of the S/O abundance ratio with respect to the cosmic S/O ratio is investigated using high-level quantum chemical simulations. The detectability of the yet to be observed analogues of both S and O molecules is also examined. From the results, the deviation from the cosmic S/O ratio is largely due to hydrogen bonding on the surface of the dust grains. As the ratio of the binding energy of S- and O-species (binding energy of S/O) with water approaches unity, the S/O abundance ratio approaches cosmic S/O ratio. The more this ratio deviates from unity, the more the S/O abundance deviates from the cosmic S/O ratio. Regarding the detectability of the unknown analogues, it suffices to say that every known O-species is an indication of the presence and detectability of the S-analogue, while for every known S-species, the O-analogue is not only present in detectable abun-

dance, it can be said to have even been overdue for astronomical detection.

**Keywords:** astrochemistry, interstellar medium, hydrogen bonding, abundance ratio,

Astrophysicists and astronomers are largely concerned with discovering new molecules in the Interstellar medium (ISM) and not so much with the understanding of the chemistry and

**Periodic Trends among Interstellar Molecular Species:** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.80884

**The Case of Oxygen- and Sulfur-Containing Species**

**The Case of Oxygen- and Sulfur-Containing Species**

Etim Emmanuel, Lawal Usman, Khanal Govinda and

Etim Emmanuel, Lawal Usman, Khanal Govinda

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80884

Mbakara Idaresit

**Abstract**

spectroscopy

**1. Introduction**

and Mbakara Idaresit

### **Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur-Containing Species Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur-Containing Species**

DOI: 10.5772/intechopen.80884

Etim Emmanuel, Lawal Usman, Khanal Govinda and Mbakara Idaresit Etim Emmanuel, Lawal Usman, Khanal Govinda and Mbakara Idaresit

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80884

### **Abstract**

Out of the 19 known S-containing interstellar molecules, 16 have the corresponding O-compound analogues marks out of the interstellar chemistry of sulfur and oxygen as a unique one among other observed interstellar periodic trends. However, the rule that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio is far from reality in many cases even when both species are observed from the same source. In this chapter, the effect of interstellar hydrogen bonding on the variation of the S/O abundance ratio with respect to the cosmic S/O ratio is investigated using high-level quantum chemical simulations. The detectability of the yet to be observed analogues of both S and O molecules is also examined. From the results, the deviation from the cosmic S/O ratio is largely due to hydrogen bonding on the surface of the dust grains. As the ratio of the binding energy of S- and O-species (binding energy of S/O) with water approaches unity, the S/O abundance ratio approaches cosmic S/O ratio. The more this ratio deviates from unity, the more the S/O abundance deviates from the cosmic S/O ratio. Regarding the detectability of the unknown analogues, it suffices to say that every known O-species is an indication of the presence and detectability of the S-analogue, while for every known S-species, the O-analogue is not only present in detectable abundance, it can be said to have even been overdue for astronomical detection.

**Keywords:** astrochemistry, interstellar medium, hydrogen bonding, abundance ratio, spectroscopy

### **1. Introduction**

Astrophysicists and astronomers are largely concerned with discovering new molecules in the Interstellar medium (ISM) and not so much with the understanding of the chemistry and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

physics of these molecules just like the early chemists were interested in discovering new chemical substances without much concern about their chemical behavior, thus leading to the emergence of the field of chemical kinetics. Understanding the chemistry of these molecules discovered by astrophysicists and astronomers has given birth to astrochemistry, a young interdisciplinary field that blends chemistry into astronomy and astrophysics. Inasmuch, we are still trying to understand the chemistry and physics of these molecules, and some of the features are very glaring to be observed by all and sundry. The dominance of organic molecules, isomerism, successive hydrogen addition, periodic trends, etc., are some of the notable features among these interstellar and circumstellar molecules. The dominance of organic molecules these molecular species is very obvious with a greater percentage of these molecules found to contain the four most important biogenic elements; C, H, N, and O. Slightly above 200 different molecular species have been detected from different astronomical sources [1]. About 132 of these species contain at least an atom of H, same number also contain at least an atom of C, 64 of these molecular species contain at least an atom of N, while not fewer than 59 contain an atom of O. The high abundances of these elements among the interstellar and circumstellar species can be seen as a direct reflection of their cosmic abundances. With the exceptions of the noble gases and the unusual abundance of Fe, these four elements (H, O, C, and N) have the highest cosmic abundances.

first detected from the same astronomical sources, suggesting a common link in their formation processes [95–97]. The abundance ratio of these molecules with respect to their cosmic or elemental abundance is also an interesting feature. According to Linke [15], "*Methyl mercaptan is apparently a fairly good example of the rule that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio*." This "rule" of course is far from being true in many cases even for molecules observed from the same source. It thus requires an in-depth investigation. Reactions that occur on the surfaces of the interstellar dust grains are the dominant processes for the formation of interstellar molecules. The composition of the interstellar dust grains, which create the surface for these reactions also serve as a platform for hydrogen bonding between the water molecule (the most abundant component of the interstellar dust grains) and the molecules that are formed on these surfaces. This interstellar hydrogen bonding, thus, reduces the abundance of molecules that are firmed on the surface of the dust grains since a greater portion of these molecules are attached to surface of the interstellar dust grains [16]. This poses a serious exception to interstellar formation processes that have been shown

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

http://dx.doi.org/10.5772/intechopen.80884

101

In the present work, the effect interstellar hydrogen bonding on the variation of the S/O abundance ratio with respect to the cosmic S/O ratio is examined using high-level quantum chemical simulations. The binding energy between water molecule on the surface of the dust grains and the O- or S-containing molecule gives inside about the level to which the interstellar abundance of such molecule is affected. There are 59 O-containing and 19 S-containing interstellar species; for 16 that are S and O analogues, there is no order regarding their astronomical observations, i.e., in some cases, the O-containing species was observed before the S-containing and vice versa. Thus, the observation of one always gives information about the presence and the possible detectability of the order. In the light of this, the known molecules from this S/O group whose corresponding analogues are not yet observed are examined for their possible detectability. These species are subjected to the effect of interstellar hydrogen bonding. Their binding energies with water on the surface of the interstellar dust grains are determined. From the ratio of the binding energies of these systems, the S/O abundance ratio is predicted for the unknown systems. For the O-containing molecules where two or more isomers are observed, standard enthalpies of formation are computed for both the O and corresponding S-analogues to guide the preference for astronomical searches for the S-analogues since the most stable isomer is more probably the most abundant in the interstellar medium except where the effect of hydrogen is well pronounced as in the case of methyl formate and acetic acid. After describing the methodology employed in this work, the results obtained are

The quantum chemical calculations reported in this work are carried out using the Gaussian 09 suite of programs [20]. The binding energy (B. E.) between the water molecule on the

to be largely thermodynamically controlled [17–19].

presented and discussed before the concluding remarks.

**2. Computational details**

Isomerism among these molecular species has emerged as one of the important tools in exploring the basic chemistry of these species. This can be understood from the fact that about 40% of all interstellar and circumstellar species have isomeric counterparts, and these isomers are believed to have a common precursor for their formation routes; thus, the detection of one isomer gives an insight about the presence and the detectability of others. That most of these isomers are easily observed from the same astronomical sources strongly supports the fact that they have a common precursor for their formation process. In the C2 H3 N isomeric group, methyl cyanide, methyl isocyanide, and ketenimine have all been observed from the same astronomical source [2–4]. In the C2 H4 O2 isomeric group, acetic acid, methyl formate, and glycolaldehyde have also been observed from the same molecular cloud [5–7]. This trend is common among isomers: HCN and HNC, MgCN and MgNC, SiCN, SiNC, etc. [8–13]. Successive hydrogen addition is considered as a possible route for the formation of alcohol from their corresponding aldehydes; methanol from formaldehyde, ethanol from acetaldehyde, and ethylene glycol from glycolaldehyde. Also, these molecules are commonly detected from the same spectral region. Laboratory experiments under interstellar medium conditions have demonstrated how small molecules grow into larger ones via successive hydrogen addition [14].

Periodic trends are another observable features of interstellar and circumstellar molecules. Elements from the same group are found to have corresponding molecules as known interstellar and circumstellar molecules as seen in the cases of C and Si, N and P, O and S, F and Cl, among others. Among these trends, those of O and S are very conspicuous. Of the 19 known S-containing molecules, 16 have the corresponding O-analogues as known interstellar and circumstellar molecules. Interestingly, 12 of the S- and their corresponding O-analogues were first detected from the same astronomical sources, suggesting a common link in their formation processes [95–97]. The abundance ratio of these molecules with respect to their cosmic or elemental abundance is also an interesting feature. According to Linke [15], "*Methyl mercaptan is apparently a fairly good example of the rule that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio*." This "rule" of course is far from being true in many cases even for molecules observed from the same source. It thus requires an in-depth investigation. Reactions that occur on the surfaces of the interstellar dust grains are the dominant processes for the formation of interstellar molecules. The composition of the interstellar dust grains, which create the surface for these reactions also serve as a platform for hydrogen bonding between the water molecule (the most abundant component of the interstellar dust grains) and the molecules that are formed on these surfaces. This interstellar hydrogen bonding, thus, reduces the abundance of molecules that are firmed on the surface of the dust grains since a greater portion of these molecules are attached to surface of the interstellar dust grains [16]. This poses a serious exception to interstellar formation processes that have been shown to be largely thermodynamically controlled [17–19].

In the present work, the effect interstellar hydrogen bonding on the variation of the S/O abundance ratio with respect to the cosmic S/O ratio is examined using high-level quantum chemical simulations. The binding energy between water molecule on the surface of the dust grains and the O- or S-containing molecule gives inside about the level to which the interstellar abundance of such molecule is affected. There are 59 O-containing and 19 S-containing interstellar species; for 16 that are S and O analogues, there is no order regarding their astronomical observations, i.e., in some cases, the O-containing species was observed before the S-containing and vice versa. Thus, the observation of one always gives information about the presence and the possible detectability of the order. In the light of this, the known molecules from this S/O group whose corresponding analogues are not yet observed are examined for their possible detectability. These species are subjected to the effect of interstellar hydrogen bonding. Their binding energies with water on the surface of the interstellar dust grains are determined. From the ratio of the binding energies of these systems, the S/O abundance ratio is predicted for the unknown systems. For the O-containing molecules where two or more isomers are observed, standard enthalpies of formation are computed for both the O and corresponding S-analogues to guide the preference for astronomical searches for the S-analogues since the most stable isomer is more probably the most abundant in the interstellar medium except where the effect of hydrogen is well pronounced as in the case of methyl formate and acetic acid. After describing the methodology employed in this work, the results obtained are presented and discussed before the concluding remarks.

### **2. Computational details**

physics of these molecules just like the early chemists were interested in discovering new chemical substances without much concern about their chemical behavior, thus leading to the emergence of the field of chemical kinetics. Understanding the chemistry of these molecules discovered by astrophysicists and astronomers has given birth to astrochemistry, a young interdisciplinary field that blends chemistry into astronomy and astrophysics. Inasmuch, we are still trying to understand the chemistry and physics of these molecules, and some of the features are very glaring to be observed by all and sundry. The dominance of organic molecules, isomerism, successive hydrogen addition, periodic trends, etc., are some of the notable features among these interstellar and circumstellar molecules. The dominance of organic molecules these molecular species is very obvious with a greater percentage of these molecules found to contain the four most important biogenic elements; C, H, N, and O. Slightly above 200 different molecular species have been detected from different astronomical sources [1]. About 132 of these species contain at least an atom of H, same number also contain at least an atom of C, 64 of these molecular species contain at least an atom of N, while not fewer than 59 contain an atom of O. The high abundances of these elements among the interstellar and circumstellar species can be seen as a direct reflection of their cosmic abundances. With the exceptions of the noble gases and the unusual abundance of Fe, these four elements (H, O, C,

Isomerism among these molecular species has emerged as one of the important tools in exploring the basic chemistry of these species. This can be understood from the fact that about 40% of all interstellar and circumstellar species have isomeric counterparts, and these isomers are believed to have a common precursor for their formation routes; thus, the detection of one isomer gives an insight about the presence and the detectability of others. That most of these isomers are easily observed from the same astronomical sources strongly supports the fact that they have a common precursor for their formation process. In the C2

isomeric group, methyl cyanide, methyl isocyanide, and ketenimine have all been observed

formate, and glycolaldehyde have also been observed from the same molecular cloud [5–7]. This trend is common among isomers: HCN and HNC, MgCN and MgNC, SiCN, SiNC, etc. [8–13]. Successive hydrogen addition is considered as a possible route for the formation of alcohol from their corresponding aldehydes; methanol from formaldehyde, ethanol from acetaldehyde, and ethylene glycol from glycolaldehyde. Also, these molecules are commonly detected from the same spectral region. Laboratory experiments under interstellar medium conditions have demonstrated how small molecules grow into larger ones via successive

Periodic trends are another observable features of interstellar and circumstellar molecules. Elements from the same group are found to have corresponding molecules as known interstellar and circumstellar molecules as seen in the cases of C and Si, N and P, O and S, F and Cl, among others. Among these trends, those of O and S are very conspicuous. Of the 19 known S-containing molecules, 16 have the corresponding O-analogues as known interstellar and circumstellar molecules. Interestingly, 12 of the S- and their corresponding O-analogues were

H4 O2 H3 N

isomeric group, acetic acid, methyl

and N) have the highest cosmic abundances.

100 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

from the same astronomical source [2–4]. In the C2

hydrogen addition [14].

The quantum chemical calculations reported in this work are carried out using the Gaussian 09 suite of programs [20]. The binding energy (B. E.) between the water molecule on the surface of the interstellar dust grains and molecule of interest (O- or S-containing) is determined using the method as described in our recent paper [94], which is expressed as Eq. (1):

B. E. (complex) = E (complex) − [E(water molecule) + E(heterocycle molecule)] (1)

To obtain high accurate values for the binding energy, the MP2(full) with the 6-311++G\*\* basis set is used in examining the effect of interstellar hydrogen bonding. By definition, the standard enthalpy of formation (∆<sup>f</sup> H0 ) of any molecule is the enthalpy change of the reaction by which it is formed from its constituent's elements. Among the different composite quantum chemical methods that are now used to accurately predict thermochemistry data, the G4 method has been found to be very effective in predicting enthalpy of formation values to chemical accuracy in many molecules as reported in literatures [17–19, 21–24]. Details regarding the steps in calculating zero-point-corrected standard enthalpy of formation have been well described in our previous studies [17–19]. The values reported in this work are calculated from the optimized geometries of the systems at the levels of theory mentioned above. The structures are found to be stationary with no imaginary frequency through harmonic vibrational frequency calculations.

### **3. Results and discussion**

The known S-containing molecules and their corresponding O-analogues are discussed with respect to the observed S/O abundance ratio followed by the detectability of the unknown analogues of these species. **Table 1** shows all the known S-containing interstellar species in a chronological order with their corresponding O-analogues (where available); the binding energies (B. E.) of these species with water on the surface of the interstellar dust grains computed at the MP2(full)/6-311++G\*\* level discussed above are presented in columns 2 and 4, respectively; for S- and O-species, the S/O ratio is from the observed abundances of these species taken from the references in the column 6. The magnitude of the binding energy shows the extent to which the molecule (S- or O-containing) is bonded to the surface of the interstellar dust grains. The higher the magnitude of the B. E., the more strongly bonded is the molecule and vice versa. This also implies that as molecule is strongly bonded to the surface of the interstellar dust grains, a greater portion of it is attached to the surface of the dust grains, thus reducing its overall abundance. When the S-containing species is more strongly bonded as compared to the O-analogue, the S/O abundance ratio becomes much more smaller than the S/O cosmic ratio of 0.024 (1/42) [15, 25] and the reverse becomes the case when O-analogue is more strongly bonded as compared to the S-analogue. When the ratio of the binding energy of an S-containing species and their O-analogue approach unity, the observed S/O ratio also approaches the cosmic S/O ratio. Because in this case, there is little or no much pronounced effect of interstellar hydrogen bonding, which affects the interstellar abundance of these species. The major exception to this trend is observed with the components of the interstellar ices: H2 O, CH3 OH, and H2 CO.

in **Table 1**. With the few exceptions observed above, S/O abundance ratio of all the known S-containing species and their corresponding O-analogues follows the same trend as displayed in **Table 2**. As the B. E. S/O ratio approaches unity, the observed S/O ratio approaches

**O-analogue B. E (kcal/mol)** 

CS (1971) −1.967 CO (1970) −0.913 0.013 [39, 52]

SiS (1975) −3.688 SiO (1971) −6.785 ≈1 [43, 44] NS (1975) 0.272 NO (1978) −0.097 0.005 [41, 42]

HNCS (1979) −7.532 HNCO (1972) −9.146 0.025 [25, 31]

HSCN (2009) −98.722 HOCN (2009) −37.227 4.5E-3 [30, 31]

SH (2012) −1.394 OH (1963) −2.927 0.023 [34, 35]

SH (2014) −1.678 CH3CH2OH (1975) −4.343 0.286 [36, 37]

**SO2 (1975) −2.123 O3 (not observed) −0.512 NA**

**SO+ (1992) −18.589 O2+ (not observed) −50.272 NA**

**C5S (2014) −1.908 C5O (not observed) −2.969 NA**

**with water**

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

(1989) −2.898 0.032 [50, 51]

O (1969) −4.672 <<0.001 [48, 49]

(2011) −0.324 0.015 [45, 46]

OH (1970) −4.417 ≈0.023 [15, 40]

(1970) −39.779 ≈0.191 [38, 39]

O (1991) −2.493 0.01 [26, 27]

O (1985) −2.584 0.028 [28, 29]

(2010) −69.343 0.029 [32, 33]

CO (1969) −4.104 ≈0.025 [47]

unity, the observed S/O ratio becomes much less than the cosmic S/O ratio, e.g., CS/CO, SO/

/HCO<sup>+</sup>

B. E. O/S ratio is inversely proportional to the observed variation of S/O abundance ratio with

*Known O-containing species and detectable S-analogues*: as previously mentioned, there are at least 59 known O-containing interstellar and circumstellar molecules of which 16 have the corresponding S-analogues as known astromolecules leaving us with over 40 O-containing species without the corresponding S-analogues. In assessing the detectability of these S-analogues of known O-containing molecules, the binding energies of these species (both S- and O-containing

S/C3

SH/CH3

CH2

O, and HSCN/HOCN and the reverse is observed when the ratio is less

CH2

, CH3

O. When this ratio is above

**S/O ratio References**

103

http://dx.doi.org/10.5772/intechopen.80884

OH. In summary, the

the cosmic S/O ratio as in the cases of HNCS/HNCO and C3

**Table 1.** S- and O-containing species, their B. E with water, and S/O ratio.

, SiS/SiO, HCS<sup>+</sup>

O2

, NS/NO, C2

**S-containing molecule**

H2

H2

CH3

HCS<sup>+</sup>

C2

C3

SH<sup>+</sup>

CH3 CH2

Ref. [15, 25–52].

**B. E (kcal/mol) with water**

OCS (1971) −1.521 CO2

S (1972) −2.292 H2

CS (1973) −2.614 H2

SH (1979) −2.048 CH3

(1981) −12.490 HCO<sup>+</sup>

S (1987) −2.602 C2

S (1987) −2.584 C3

(2011) −73.314 OH<sup>+</sup>

SO (1973) −3.063 O2

the cosmic S/O ratio.

S/C2

than unity, e.g., OCS/CO2

which are thus more abundant than their corresponding S-analogues irrespective of the effect of interstellar hydrogen bonding. **Figure 1** and **Table 2** summarize the observed trends Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur… http://dx.doi.org/10.5772/intechopen.80884 103


**Table 1.** S- and O-containing species, their B. E with water, and S/O ratio.

surface of the interstellar dust grains and molecule of interest (O- or S-containing) is determined using the method as described in our recent paper [94], which is expressed as Eq. (1):

To obtain high accurate values for the binding energy, the MP2(full) with the 6-311++G\*\* basis set is used in examining the effect of interstellar hydrogen bonding. By definition, the

by which it is formed from its constituent's elements. Among the different composite quantum chemical methods that are now used to accurately predict thermochemistry data, the G4 method has been found to be very effective in predicting enthalpy of formation values to chemical accuracy in many molecules as reported in literatures [17–19, 21–24]. Details regarding the steps in calculating zero-point-corrected standard enthalpy of formation have been well described in our previous studies [17–19]. The values reported in this work are calculated from the optimized geometries of the systems at the levels of theory mentioned above. The structures are found to be stationary with no imaginary frequency through harmonic vibra-

The known S-containing molecules and their corresponding O-analogues are discussed with respect to the observed S/O abundance ratio followed by the detectability of the unknown analogues of these species. **Table 1** shows all the known S-containing interstellar species in a chronological order with their corresponding O-analogues (where available); the binding energies (B. E.) of these species with water on the surface of the interstellar dust grains computed at the MP2(full)/6-311++G\*\* level discussed above are presented in columns 2 and 4, respectively; for S- and O-species, the S/O ratio is from the observed abundances of these species taken from the references in the column 6. The magnitude of the binding energy shows the extent to which the molecule (S- or O-containing) is bonded to the surface of the interstellar dust grains. The higher the magnitude of the B. E., the more strongly bonded is the molecule and vice versa. This also implies that as molecule is strongly bonded to the surface of the interstellar dust grains, a greater portion of it is attached to the surface of the dust grains, thus reducing its overall abundance. When the S-containing species is more strongly bonded as compared to the O-analogue, the S/O abundance ratio becomes much more smaller than the S/O cosmic ratio of 0.024 (1/42) [15, 25] and the reverse becomes the case when O-analogue is more strongly bonded as compared to the S-analogue. When the ratio of the binding energy of an S-containing species and their O-analogue approach unity, the observed S/O ratio also approaches the cosmic S/O ratio. Because in this case, there is little or no much pronounced effect of interstellar hydrogen bonding, which affects the interstellar abundance of these species. The major exception to this trend

which are thus more abundant than their corresponding S-analogues irrespective of the effect of interstellar hydrogen bonding. **Figure 1** and **Table 2** summarize the observed trends

H0

standard enthalpy of formation (∆<sup>f</sup>

102 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

tional frequency calculations.

**3. Results and discussion**

is observed with the components of the interstellar ices: H2

B. E. (complex) = E (complex) − [E(water molecule) + E(heterocycle molecule)] (1)

) of any molecule is the enthalpy change of the reaction

O, CH3

OH, and H2

CO.

in **Table 1**. With the few exceptions observed above, S/O abundance ratio of all the known S-containing species and their corresponding O-analogues follows the same trend as displayed in **Table 2**. As the B. E. S/O ratio approaches unity, the observed S/O ratio approaches the cosmic S/O ratio as in the cases of HNCS/HNCO and C3 S/C3 O. When this ratio is above unity, the observed S/O ratio becomes much less than the cosmic S/O ratio, e.g., CS/CO, SO/ O2 , NS/NO, C2 S/C2 O, and HSCN/HOCN and the reverse is observed when the ratio is less than unity, e.g., OCS/CO2 , SiS/SiO, HCS<sup>+</sup> /HCO<sup>+</sup> , CH3 CH2 SH/CH3 CH2 OH. In summary, the B. E. O/S ratio is inversely proportional to the observed variation of S/O abundance ratio with the cosmic S/O ratio.

*Known O-containing species and detectable S-analogues*: as previously mentioned, there are at least 59 known O-containing interstellar and circumstellar molecules of which 16 have the corresponding S-analogues as known astromolecules leaving us with over 40 O-containing species without the corresponding S-analogues. In assessing the detectability of these S-analogues of known O-containing molecules, the binding energies of these species (both S- and O-containing

**O-containing molecule**

N2

H2

H3

H2 C2

H2

CH3

H2

H2

HC2

c-H2 C3

CH3

c-C2 H4

HNO 6E11–

O<sup>+</sup> 7.2E12,

**Column density (cm−<sup>2</sup> )**

O ≈E15 [60] −2.225 N2

3.2E14

2.3E13, 1.1E15

O<sup>+</sup> 3E14 [68] −30.591 H3

O ≈E14 [72] −2.191 H2

O 7E11 [75] −3.180 CH3

NCO<sup>+</sup> 6-14E11 [76] −21.372 H2

NCHO 2.2E16 [77] −5.457 H2

CHO 1.5E12 [78] −4.079 HC2

CHO ≈1.5E14 [80] −4.675 CH3

O ≈E13 [79] −6.081 c-H2

O 3.3E14 [81] −4.457 c-C2

COH<sup>+</sup> E12-E14 [73] −25.388 H2

**Refs. B. E. (kcal/ mol) with water**

CO<sup>+</sup> ≈E12 [53] −58.540 CS<sup>+</sup> −13.562 0.232 >S/O\* FeO 9E11 [54] 10.299 FeS −3.637 0.353 >S/O\* PO ≈2.8E15 [55] −10.236 PS −1.228 0.120 >S/O\* OH<sup>+</sup> 2.4E15 [56] −69.343 SH<sup>+</sup> −73.314 1.057 ≈S/O\* TiO 6.99E14 [57] −3.874 TiS −2.413 0.623 >S/O\* NO<sup>+</sup> 2.2E12 [58] −20.719 NS<sup>+</sup> −21.547 1.040 ≈S/O\* AlO ≈2E15 [59] −0.027 AlS −15.032 556.741 <S/O\*

HCO ≈E11 [61] −27.168 HCS −7.477 0.275 >S/O\*

HOC<sup>+</sup> ≈3E12 [63] −72.861 HSC<sup>+</sup> −127.024 1.743 <S/O\* OCN<sup>−</sup> — [64] −11.887 SCN<sup>−</sup> −9.036 0.760 >S/O\*

TiO2 7.5E14 [57] −32.491 TiOS −15.607 0.480 >S/O\* HO2 2.8E12 [66] −2.066 HSO −2.675 1.295 ≈S/O\* AlOH ≈E17 [67] −4.043 AlSH −21.869 5.409 <S/O\*

HOCO<sup>+</sup> — [69] −33.959 HOCS<sup>+</sup> −45.806 1.349 ≈S/O\* HCNO ≈8.9E12 [56] −1.948 HCNS −1.148 0.589 >S/O\* HOOH 8E12 [70] −5.894 HOSH −2.747 0.466 >S/O\* HCOOH ≈5E13 [71] −4.353 HSCHO −0.436 0.100 >S/O\*

CNCHO 1-17E14 [74] −4.743 CNCHS −4.201 0.887 ≈S/O\*

C2

C3

H4

[65] −45.608 H2

**S-analogue B. E. (kcal/**

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

[62] −72.436 HNS −35.794 0.494 >S/O\*

**mol) with water**

S −1.862 0.837 ≈S/O\*

S<sup>+</sup> −19.086 0.418 >S/O\*

S<sup>+</sup> −17.753 0.580 >S/O\*

S −0.486 0.222 >S/O\*

CSH<sup>+</sup> −15.103 0.595 >S/O\*

S −2.175 0.684 >S/O\*

NCS<sup>+</sup> −17.922 0.838 ≈S/O\*

NCHS −4.171 0.764 >S/O\*

CHS −2.908 0.713 >S/O\*

CHS −2.544 0.544 >S/O\*

S −5.628 0.925 ≈S/O\*

S −2.892 0.689 >S/O\*

**B. E. S/O ratio**

http://dx.doi.org/10.5772/intechopen.80884

**Estimated S/O abundance ratio**

105

**Figure 1.** Correlation between B. E. and S/O abundance ratio.


**Table 2.** Deviation from cosmic S/O ratio as a function of binding energy (B. E.).

species) with water on the surface of the interstellar dust grains have been computed. These values are presented in **Table 3**. The reported column densities for the known O-containing molecules are shown in the column 2 with the source of the data in the column 3 of the same table (refs). The column 7 shows the ratio of the binding energy of the S- and O-containing species, from this ratio, the S/O abundance ratio is predicted (column 8) following the observations made in the preceding section (see **Table 2**). That the S-containing molecular species are less bonded to the surface of the interstellar dust grains compared to their respective O-analogues as it is observed in over 80% of the systems here (**Table 3**) is a good omen with respect to the detectability of these species because their overall interstellar abundance will be less affected by the effect of interstellar hydrogen bond unlike their O-analogues. However, with respective to the role that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio, there will be much deviation from this role since the degree to which the S-containing species is affected by the effect of hydrogen bonding on the surface of the dust grains is much different from those of the corresponding O-analogues. As a result of this, S/O abundance ratio would be expected to be much higher than the cosmic S/O ratio as shown in the column 8 of **Table 1** for majority of the cases and in very few cases the ratio will tend toward the cosmic S/O ratio except where other processes play a role.

Interstellar formation processes have been shown to be largely thermodynamically controlled in many cases. Except with a pronounced effect of interstellar hydrogen bonding, the most Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur… http://dx.doi.org/10.5772/intechopen.80884 105


species) with water on the surface of the interstellar dust grains have been computed. These values are presented in **Table 3**. The reported column densities for the known O-containing molecules are shown in the column 2 with the source of the data in the column 3 of the same table (refs). The column 7 shows the ratio of the binding energy of the S- and O-containing species, from this ratio, the S/O abundance ratio is predicted (column 8) following the observations made in the preceding section (see **Table 2**). That the S-containing molecular species are less bonded to the surface of the interstellar dust grains compared to their respective O-analogues as it is observed in over 80% of the systems here (**Table 3**) is a good omen with respect to the detectability of these species because their overall interstellar abundance will be less affected by the effect of interstellar hydrogen bond unlike their O-analogues. However, with respective to the role that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio, there will be much deviation from this role since the degree to which the S-containing species is affected by the effect of hydrogen bonding on the surface of the dust grains is much different from those of the corresponding O-analogues. As a result of this, S/O abundance ratio would be expected to be much higher than the cosmic S/O ratio as shown in the column 8 of **Table 1** for majority of the cases and in very few cases the ratio will tend

S/O B. E. <1

S/O ratio

>Cosmic S/O ratio

Interstellar formation processes have been shown to be largely thermodynamically controlled in many cases. Except with a pronounced effect of interstellar hydrogen bonding, the most

toward the cosmic S/O ratio except where other processes play a role.

**Figure 1.** Correlation between B. E. and S/O abundance ratio.

104 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

S/O B. E. ≈1

S/O ratio

**Table 2.** Deviation from cosmic S/O ratio as a function of binding energy (B. E.).

≈Cosmic S/O ratio

S/O B. E. >1

S/O ratio

<Cosmic S/O ratio


**Figure 2.** Dependence of column density on enthalpy of formation for CHNO and CHNS systems.

**3-atoms** HXC<sup>+</sup> 234.419 340.747 HCX<sup>+</sup> 198.564 246.625 **4-atoms**

HCNX 34.084 61.162 HXCN −4.387 38.312 HNCX −33.357 27.126

**7-atoms**

CX −14.596 19.146

CCHXH (anti) −28.519 19.386

CCHXH (syn) −30.236 19.439

CCHX −42.405 16.453 **8-atoms**

CCHX −70.542 31.962

CXCHX −89.381 20.743

C(X)XH −103.746 18.612 **9-atoms**

X −48.956 −10.697

XH −56.718 −11.943 **11 atoms**

**X = O X = S**

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

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107

**Molecule Enthalpy of formation (kcal/mol)**

c-H2 CH2

H2

H2

H3

HXH2

H3

H3

(CH3 ) 2

C2 H5

Refs. [7, 17, 53–89].

**Table 3.** Parameters for known O- and detectable S-containing molecules.

stable isomer has always been observed to be the most abundant isomer in the interstellar space. Thus, the most stable isomer is easily observed as compared to other isomers of the group. **Figure 2** pictures this concept. It shows how the interstellar abundance (column density) of two isomers each from the CHNO and CHNS groups varies with the stability (enthalpy of formation) where the most stable isomer (with lower enthalpy of formation) is found to be the most abundant in both cases. Searching for the most stable isomer is, thus, a step toward successful observation, and the successful detection of an isomer reaffirms the presences of other isomers since they are believed to have a common precursor for their formation routes. In view of this, for known O-containing molecules with at least two isomers, the standard enthalpies of formation for these isomers and their S-analogues have been determined as a guide for preference in the astronomical searches for these isomers. **Table 4** presents the enthalpy of formation for O-containing isomers and their detectable S-analogues. As would be expected, the trend of the stability for O and S-species is the same. From the parameters presented in **Table 2** coupled with the advancements in astronomical and spectroscopic equipment, that all the S-analogues of known O-containing interstellar molecular species would not be considered as exaggeration. They are detectable.

*Known S-species and overdue detectable O-analogue*: without any exception, an interstellar O-containing molecular species is more abundant than its S-analogue (**Table 1**). So for every Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur… http://dx.doi.org/10.5772/intechopen.80884 107

**Figure 2.** Dependence of column density on enthalpy of formation for CHNO and CHNS systems.

stable isomer has always been observed to be the most abundant isomer in the interstellar space. Thus, the most stable isomer is easily observed as compared to other isomers of the group. **Figure 2** pictures this concept. It shows how the interstellar abundance (column density) of two isomers each from the CHNO and CHNS groups varies with the stability (enthalpy of formation) where the most stable isomer (with lower enthalpy of formation) is found to be the most abundant in both cases. Searching for the most stable isomer is, thus, a step toward successful observation, and the successful detection of an isomer reaffirms the presences of other isomers since they are believed to have a common precursor for their formation routes. In view of this, for known O-containing molecules with at least two isomers, the standard enthalpies of formation for these isomers and their S-analogues have been determined as a guide for preference in the astronomical searches for these isomers. **Table 4** presents the enthalpy of formation for O-containing isomers and their detectable S-analogues. As would be expected, the trend of the stability for O and S-species is the same. From the parameters presented in **Table 2** coupled with the advancements in astronomical and spectroscopic equipment, that all the S-analogues of known O-containing interstellar molecular

*Known S-species and overdue detectable O-analogue*: without any exception, an interstellar O-containing molecular species is more abundant than its S-analogue (**Table 1**). So for every

species would not be considered as exaggeration. They are detectable.

**O-containing molecule**

CH2

CH3

HOCH2

CH2

(NH2 )2

CH3

CH3

(CH3 )2

CH3 CH2

C2 H5

CH3

C2 H5

Refs. [7, 17, 53–89].

HOCH2 CH2 **Column density (cm−<sup>2</sup> )**

CHOH 2.4E13 [82] −6.103 CH2

106 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

COOH 7.3E15 [6] −7.863 CH3

CHCHO — [83] −5.215 CH2

OCH3 <18E14 [85] −4.411 CH3

CONH2 1.8E14 [86] −5.922 CH3

CO 2.9E16 [87] −5.051 (CH3

CHO — [83] −4.560 CH3

OCHO 5.4E16 [88] −4.896 C2

COOCH3 4.2E15 [88] −4.815 CH3

OCH3 2E14 [89] −4.250 C2

**Table 3.** Parameters for known O- and detectable S-containing molecules.

Where S/O\* is the cosmic abundance ratio.

OH 3.2E14 [14] −4.064 HOCH2

CO ≈E15 [84] −7.422 (NH2

CHO 2.8E16 [7] −5.414 HSCH2

**Refs. B. E. (kcal/ mol) with water**

HCOOCH3 ≈1.9E17 [7] −4.975 HCSOCH3 −2.418 0.486 >S/O\*

)2

)2

CH2

H5

H5

CH2

**S-analogue B. E. (kcal/**

**mol) with water**

CSOH −7.001 0.890 ≈S/O\*

CHO −4.337 0.801 >S/O\*

CS −6.676 0.899 ≈S/O\*

CHCHS −2.843 0.545 >S/O\*

SCH3 −2.548 0.578 >S/O\*

CSNH2 −3.914 0.661 >S/O\*

CS −6.711 1.329 ≈S/O\*

CHS −3.296 0.723 >S/O\*

OCHO −14.803 3.023 <S/O\*

COOCH3 −4.006 0.831 >S/O\*

OCH3 −2.519 0.593 >S/O\*

SH −2.073 0.510 >S/O\*

CHSH −2.585 0.423

**B. E. S/O ratio**

**Estimated S/O abundance ratio**



**Table 4.** Enthalpy of formation for O-containing isomers and their detectable S-analogues.


1 1 H,Protium (H), 1

hot coke.

with zinc.

**Uses of hydrogen:**

**Chemical properties of oxygen:**

Hydrogen gas is made up of hydrogen molecule, H2

2

Colorless, odorless, and tasteless gas.

1

**4.2. Large-scale production of hydrogen**

chain hydrocarbons into smaller molecules.

H,deuterium (D), and 1

Tritium is radioactive, decaying by beta emission.

3

Density at s.t.p 0.09 KJmol−<sup>1</sup> Bond energy, H–H 436 KJmol−<sup>1</sup> Bond length, H–H 74 pm

Molecular formula H2 Melting point 14 K Boiling point 20 K

2H + <sup>1</sup>

The result is a mixture of carbon dioxide, carbon monoxide, and hydrogen.

CH4(g) + H<sup>2</sup>

CO(g) + H<sup>2</sup>

C6

H12(g) → C6

of margarines from vegetable oils, welding, and fuels cells are some of its other uses.

The naturally occurring isotope is deuterium, while tritium is made during nuclear reaction.

2H → <sup>1</sup>

Natural gas (methane) is an important source of hydrogen. Methane is reacted with steam at high pressure of about 35 atmosphere pressure and 800°C in the presence of a nickel catalyst.

In the refining industry, hydrogen is obtained in many reactions that involve cracking long-

Bosch reaction is another method of making hydrogen; here, steam is passed over white

Majority of hydrogen produced are used in making ammonia in the Haber process. Production

on their own. The explosive mixture of hydrogen and oxygen is its commonest reaction. The possibility of explosion exists in the laboratory when hydrogen is made in large scale, and the experiment mostly goes on with caution. The normal method is to react dilute sulfuric acid

3H + <sup>1</sup>

H,tritium (T) are the three known isotopes of hydrogen.

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

O(g) → CO(g) + 3H2(g) (3)

O(g) → CO(g) + H2(g) (4)

H6(g) + 3H2(g) (5)

. Hydrogen atoms are too reactive to exist

1H (2)

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109

Refs. [90–93].

**Table 5.** Parameters for known S- and detectable O-containing molecules.

known S-species, the O-analogue is not only present in detectable abundance, it can be said to have even been overdue for astronomical detection because for sure the O-species are more abundant than their S-analogue and as such could be detected with less difficulty as compared to its S-analogue. **Table 5** lists the parameters for known S-containing interstellar species and their detectable O-analogues. The high abundances reported for these S-containing species (column 2) strongly support the detectability of their O-analogues.

### **4. Basic inorganic chemistry of oxygen, sulfur, and hydrogen**

### **4.1. Hydrogen**

The first element in the periodic table is hydrogen. Although hydrogen is very abundant in nature, the air contains almost no free hydrogen. On the other hand, hydrogen is found in water, which is about 70% of the earth. Additionally, hydrogen compounds combined with carbon are found in space and it plays important roles in such as the nuclear fission reaction in the sun. Molecules of associated hydrogen element are the basis of astrochemical space research.

Physical properties of hydrogen



1 1 H,Protium (H), 1 2 H,deuterium (D), and 1 3 H,tritium (T) are the three known isotopes of hydrogen. The naturally occurring isotope is deuterium, while tritium is made during nuclear reaction.

$$\text{H}\_{1}^{1} + \text{H}\_{1}^{3} \leftarrow \text{H}\_{1}^{2} + \text{H}\_{1}^{2} + \text{H}\_{1}^{3}$$

Tritium is radioactive, decaying by beta emission.

### **4.2. Large-scale production of hydrogen**

Natural gas (methane) is an important source of hydrogen. Methane is reacted with steam at high pressure of about 35 atmosphere pressure and 800°C in the presence of a nickel catalyst. The result is a mixture of carbon dioxide, carbon monoxide, and hydrogen.

$$\text{CH}\_{4(g)} + \text{H}\_2\text{O}\_{(g)} \rightarrow \text{CO}\_{(g)} + 3\text{H}\_{2(g)}\tag{3}$$

$$\text{CO}\_{\text{(g)}} + \text{H}\_{2}\text{O}\_{\text{(g)}} \rightarrow \text{CO}\_{\text{(g)}} + \text{H}\_{2\text{(g)}}\tag{4}$$

In the refining industry, hydrogen is obtained in many reactions that involve cracking longchain hydrocarbons into smaller molecules.

$$\text{C}\_6\text{H}\_{12(g)} \rightarrow \text{C}\_6\text{H}\_{\epsilon(g)} + 3\text{H}\_{2(g)}\tag{5}$$

Bosch reaction is another method of making hydrogen; here, steam is passed over white hot coke.

#### **Uses of hydrogen:**

known S-species, the O-analogue is not only present in detectable abundance, it can be said to have even been overdue for astronomical detection because for sure the O-species are more abundant than their S-analogue and as such could be detected with less difficulty as compared to its S-analogue. **Table 5** lists the parameters for known S-containing interstellar species and their detectable O-analogues. The high abundances reported for these S-containing species

**O-analogue B. E. (kcal/**

**X = O X = S**

**mol) with water**

<sup>+</sup> −50.272 0.370 >S/O\*

O −2.969 0.643 >S/O\*

**B. E. S/O ratio**

**Estimated S/O abundance ratio**

The first element in the periodic table is hydrogen. Although hydrogen is very abundant in nature, the air contains almost no free hydrogen. On the other hand, hydrogen is found in water, which is about 70% of the earth. Additionally, hydrogen compounds combined with carbon are found in space and it plays important roles in such as the nuclear fission reaction in the sun. Molecules of associated hydrogen element are the basis of astrochemical space research.

(column 2) strongly support the detectability of their O-analogues.

Relative atomic mass 1.008 Electron structure 1s1

Ionization energy 1312 KJmol−<sup>1</sup> Electron affinity −72 KJmol−<sup>1</sup>

**References B. E. (kcal/**

**Table 4.** Enthalpy of formation for O-containing isomers and their detectable S-analogues.

COC(O)CH3 −95.098 12.071

CXCHX −97.515 12.489

**Molecule Enthalpy of formation (kcal/mol)**

108 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

**mol) with water**

SO2 <3.5E16 93 −2.123 O3 −0.512 4.146 <S/O\*

**4.1. Hydrogen**

**S-containing molecule**

Refs. [90–93].

C5

H3

H3 CH2

> **Column density (cm−<sup>2</sup> )**

Where S/O\* is the cosmic abundance ratio.

SO<sup>+</sup> 5E12 94 −18.589 O2

S 2-14E12 95 −1.908 C5

**Table 5.** Parameters for known S- and detectable O-containing molecules.

Physical properties of hydrogen

**4. Basic inorganic chemistry of oxygen, sulfur, and hydrogen**

Majority of hydrogen produced are used in making ammonia in the Haber process. Production of margarines from vegetable oils, welding, and fuels cells are some of its other uses.

#### **Chemical properties of oxygen:**

Hydrogen gas is made up of hydrogen molecule, H2 . Hydrogen atoms are too reactive to exist on their own. The explosive mixture of hydrogen and oxygen is its commonest reaction. The possibility of explosion exists in the laboratory when hydrogen is made in large scale, and the experiment mostly goes on with caution. The normal method is to react dilute sulfuric acid with zinc.

$$\rm Zn\_{(s)} + 2H^{+}\_{\text{(aq)}} \to Zn^{2+} + H\_{2(g)} \tag{6}$$

Furthermore, molecular orbital theory clarifies situations like this. Here, bridging hydrogen atoms are believed to be bonded to the boron atoms by a bond stretching across all three atoms. This type of bond is called a three center bond. Each one contains two electrons, which together with the four pairs of electrons in the bonds to the four terminal hydrogen atoms, bringing the total to the twelve (12) electrons observed. The bonding in other members of the borane group is explained in similar fashion, although bigger members show more complex-

(lithium aluminum hydride), is another hydride

http://dx.doi.org/10.5772/intechopen.80884

. The importance of oxygen to plant and

O and sharing of electrons like in the case

O<sup>+</sup> , R3 O<sup>+</sup> , etc. 17O and 8 18O 111

, oxygen alone constitutes about 20.8% of air. Three isotopes of oxy-

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

16O is the main isotope with an abundance of 99.8%. The others are 8

with the abundance of 0.04 and 0.2%, respectively. Oxygen is very important to life of animals and plants including human being on earth. Oxygen is slightly soluble in water, which is

Moreover, oxygen is an essential component of combustion reaction, especially of organic materials such as wood, oil, and coal. Our modern age is characterized by combustion of these

Oxygen is located at upper layer of atmosphere where it absorbs harmful ultraviolet light

Oxygen compounds of all elements are known except for those of He, Ne, and Ar. Molecular

The chemistry of oxygen is representative of having the neon stable configuration in the fol-

The varieties of physical and chemical properties showed by oxides are functions of the bond types from primarily electrovalent to covalent. Formation of oxide ion is an energy consum-

In ionic metal oxide formation, energy is also expanded in vaporizing and ionization of metal atom. Many ionic oxides are possible as a result of the high lattice energy of oxides that have the lesser double bond charged O2− ion. Where this lattice energy is not sufficient to give the needed energy for ionization, oxides with significant covalent attributes are formed. The fol-

, etc.

(dioxygen) react with all elements in periodic table with the exception of the halo-

ity in their bonding.

rather being unusual.

A diatomic molecule, O2

**4.4. Oxygen**

oxygen O2

**4.5. Oxides**

ing process.

gen are known; 8

Lithium tetrahydridoaluminate (III), LiAlH4

essential for fish and other aquatic life survival.

fuels for our electricity, transportation, and heating needs.

from the sun. Ozone is an allotrope of oxygen, O3

lowing ways. Gaining of electron ad in the case of H2

of OH−, and finally formation three covalent bonds like in H<sup>3</sup>

animal life cannot be overemphasized.

lowing are some examples BeO, SiO2

gens, some noble metals, and the rare gases.

Hydrogen forms hydride with many elements. Hydrides of metals are ionic, example NaH and CaH2 . Hydrides of nonmetals are covalent in nature, example CH4 and NH4 . Hydrogen forms hydrogen bonding with highly electronegative atoms, for example HF and H2 O. Hydrogen gas explodes with oxygen when ignited.

$$\text{2H}\_{2(g)} + \text{O}\_{2(g)} \to \text{2H}\_2\text{O}\_{(g)}\tag{7}$$

As a reducing agent, hydrogen will remove oxygen from many oxides.

$$\text{CuO}\_{\text{(s)}} + \text{H}\_{\text{2(g)}} \to \text{Cu}\_{\text{(s)}} + \text{H}\_{\text{2}}\text{O}\_{\text{(g)}}\tag{8}$$

Hydrogen is liberated from acids by metals.

$$\text{Zn}\_{\text{(8)}} + 2\text{H}^{+} \rightarrow \text{Zn}^{2+} \_{\text{(aq)}} + \text{H}\_{2\text{(g)}} \tag{9}$$

Hydrogen is prepared in the laboratory this way.

Hydrogen can exist both as H<sup>+</sup> (aq) and H3 O(aq) in water. Hydrogen ions form the active ions in aqueous aids.

#### **4.3. Unusual hydrides of hydrogen**

Chemical bonding theories were unable to explain chemical bonding in boron hydrides when they were first examined. They are several boron hydrides some of which are shown below.


The simplest member of the group is diborane. The six hydrogen atoms provide six electrons for bonding, and there are three valence electrons in the shell of each work, with 12 electrons in all. However, X-ray structure of diborane reveals that each boron atom has two hydrogen atoms attached to it and another two hydrogen atoms shared between the two boron atoms. Expectedly, there will be sixteen (16) electrons participating in eight (8) bonds. Conversely, this is not the case as there appear to be too few electrons accounting for the number of bonds. Such molecules are now known as the electron-deficient molecules or compounds.

Furthermore, molecular orbital theory clarifies situations like this. Here, bridging hydrogen atoms are believed to be bonded to the boron atoms by a bond stretching across all three atoms. This type of bond is called a three center bond. Each one contains two electrons, which together with the four pairs of electrons in the bonds to the four terminal hydrogen atoms, bringing the total to the twelve (12) electrons observed. The bonding in other members of the borane group is explained in similar fashion, although bigger members show more complexity in their bonding.

Lithium tetrahydridoaluminate (III), LiAlH4 (lithium aluminum hydride), is another hydride rather being unusual.

### **4.4. Oxygen**

Zn(s) + 2H+

hydrogen bonding with highly electronegative atoms, for example HF and H2

. Hydrides of nonmetals are covalent in nature, example CH4

As a reducing agent, hydrogen will remove oxygen from many oxides.

(aq) and H3

gas explodes with oxygen when ignited.

110 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

Hydrogen is liberated from acids by metals.

Hydrogen can exist both as H<sup>+</sup>

Diborane B2

Tetraborane B4

Hexaborane-10 B6

**4.3. Unusual hydrides of hydrogen**

**Name Formula Comment**

aqueous aids.

ecules or compounds.

Hydrogen is prepared in the laboratory this way.

CaH2

Hydrogen forms hydride with many elements. Hydrides of metals are ionic, example NaH and

2H2(g) + O2(g) → 2H2

CuO(s) + H2(g) → Cu(s) + H<sup>2</sup>

Chemical bonding theories were unable to explain chemical bonding in boron hydrides when they were first examined. They are several boron hydrides some of which are shown below.

H6 Highly flammable and hydrolyzed easily

H10 Less reactive than diborane

Decaborane B10H14 Does not easily reacts with air or water Icosaborane-10 B20H16 Does not easily reacts with air or water

H10 Same reactivity like tetraborane

The simplest member of the group is diborane. The six hydrogen atoms provide six electrons for bonding, and there are three valence electrons in the shell of each work, with 12 electrons in all. However, X-ray structure of diborane reveals that each boron atom has two hydrogen atoms attached to it and another two hydrogen atoms shared between the two boron atoms. Expectedly, there will be sixteen (16) electrons participating in eight (8) bonds. Conversely, this is not the case as there appear to be too few electrons accounting for the number of bonds. Such molecules are now known as the electron-deficient mol-

Zn(S) + 2H+ → Zn2+

(aq) → Zn2+ + H2(g) (6)

and NH4

O(g) (7)

O(g) (8)

(aq) + H2(g) (9)

O(aq) in water. Hydrogen ions form the active ions in

. Hydrogen forms

O. Hydrogen

A diatomic molecule, O2 , oxygen alone constitutes about 20.8% of air. Three isotopes of oxygen are known; 8 16O is the main isotope with an abundance of 99.8%. The others are 8 17O and 8 18O with the abundance of 0.04 and 0.2%, respectively. Oxygen is very important to life of animals and plants including human being on earth. Oxygen is slightly soluble in water, which is essential for fish and other aquatic life survival.

Moreover, oxygen is an essential component of combustion reaction, especially of organic materials such as wood, oil, and coal. Our modern age is characterized by combustion of these fuels for our electricity, transportation, and heating needs.

Oxygen is located at upper layer of atmosphere where it absorbs harmful ultraviolet light from the sun. Ozone is an allotrope of oxygen, O3 . The importance of oxygen to plant and animal life cannot be overemphasized.

Oxygen compounds of all elements are known except for those of He, Ne, and Ar. Molecular oxygen O2 (dioxygen) react with all elements in periodic table with the exception of the halogens, some noble metals, and the rare gases.

The chemistry of oxygen is representative of having the neon stable configuration in the following ways. Gaining of electron ad in the case of H2 O and sharing of electrons like in the case of OH−, and finally formation three covalent bonds like in H<sup>3</sup> O<sup>+</sup> , R3 O<sup>+</sup> , etc.

### **4.5. Oxides**

The varieties of physical and chemical properties showed by oxides are functions of the bond types from primarily electrovalent to covalent. Formation of oxide ion is an energy consuming process.

In ionic metal oxide formation, energy is also expanded in vaporizing and ionization of metal atom. Many ionic oxides are possible as a result of the high lattice energy of oxides that have the lesser double bond charged O2− ion. Where this lattice energy is not sufficient to give the needed energy for ionization, oxides with significant covalent attributes are formed. The following are some examples BeO, SiO2 , etc.

### **4.6. Sources of oxygen**

Fractional distillation of air is the main source of obtaining oxygen. Air is forced under pressure through nozzles. The compressed air is allowed to expand into a region of lower pressure, which cools the air. The air is cooled further in expansion tubes until it condenses into liquid. The liquid air is a mixture of nitrogen, oxygen, and rare gases and is separated by allowing an increase in temperature of the medium. The other gases boil more easily than oxygen, so they evaporate leaving oxygen. Liquid oxygen, which is pale blue in color and strongly paramagnetic, is stored under pressure or in insulated containers.

Oxygen extracted this way is used to aid respiration medically, for breathing by divers and astronauts, in oxy-acetylene welding and rocket fuels, etc.

#### **4.7. Ozone**

An allotrope of oxygen after dioxygen (O2 ). Ozone (O3 ) is formed by the action of electric current on oxygen, concentration of about 10% realized this way. Ozone is blue in color like oxygen but diamagnetic. Pure ozone is a deep blue explosive liquid, which is obtained by fractional liquefaction of O2 –O3 mixture. In the atmosphere, ozone comes about by the action of ultraviolet light radiation on oxygen. Ozone located at about the altitude of 25 km is responsible for preventing excess ultraviolet light from reaching on the earth.

Ozone is chemically found to be very endothermic and decomposes only slowly at 250°C in the presence of catalyst or ultraviolet (UV) light.

$$\mathcal{O}\_{\mathfrak{z}} \rightleftharpoons \frac{\mathfrak{z}}{2} \mathcal{O}\_{\mathfrak{z}} \Delta \mathcal{H} = -142 \text{ kJ} \text{mol}^{-1} \tag{10}$$

O2(g) + O(g) → O3(g) (13)

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113

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

This reaction is of extreme importance to the maintenance of balance here on earth. This is because the ozone has the ability to absorb dangerous ultraviolet radiation from the sun, thus preventing it from reaching the earth surface. This radiation is of high energy, therefore, of short wavelength. If too much of this radiation reaches the earth, the energy balance will be

Secondly, exposure to increase ultraviolet radiation will lead to cell mutation of living tissues.

The ability of oxygen molecule to combine with both metals and nonmetals to form oxides is its most outstanding property. Oxides are of four different types: neutral, basic, acidic, and amphoteric. Oxides that show both basic and acidic behavior are called amphoteric oxides. Few oxides are neutral: nitrous oxide and carbon monoxide are examples. Group I and II oxides are examples of basic oxides. Oxides of nonmetals are mainly acidic in nature.

(aq) → 2Al3+

colorless liquid with a boiling point of 152.1 °C. It is similar to water in many of its properties

(aq) + 3H<sup>2</sup>

O(l) → 2Al(OH)6

3−

O5 O2 F2

O10 SO2 Cl2

O3 TeO2 I2

O2 PoO2

O3 SeO2 BrO

O2

O(l) (14)

(aq) (15)

O

O5

O

. Hydrogen peroxide is a

upset leading to increase greenhouse effect and global atmospheric temperature.

The consequences of this will increase in skin cancer incidence in particular.

**4.8. Chemical properties of oxygen**

Aluminum oxide is an example of amphoteric oxide.

It shows basic properties by reacting with hydrogen ions

Al2

Formulae of some oxides in the periodic table:

O BeO B2

O MgO Al2

O CaO Ga2

O SrO In2

O BaO Tl2

Not all the oxides are shown.

Al2

Additionally, it shows acidic property when dissolved in alkali

The most important peroxide of oxygen is hydrogen peroxide, H2

O3(s) + 6H+

O3(s) + 6OH<sup>−</sup> + 3H<sup>2</sup>

O3 CO2 N2

O3 SiO2 P4

O3 GeO2 As2

O3 SnO Sb2

O PbO Bi2

**Group I Group II Group III Group IV Group V Group VI Group VII**

*4.8.1. Oxides*

*4.8.2. Peroxides*

Li2

Na2

K2

Rb2

Cs2

Ozone is triangular in shape with equal bond length of about 128 nm. The degree of single and double bonds formed by ozone is same. Each oxygen atom of the ozone has six valence electrons. Describing this with valence bond theory, each bond will involve a set of resonance hybrid in which one of the bonds is a double bond and the other is a coordinate bond. Moreover, the real structure does not swap between the resonance frames; rather, each bond partly shows a nature of a double bond and partly that of a single bond.

In the laboratory, ozone is made by passing oxygen through an electric field. An equilibrium is set up.

$$\text{\textsf{2O}}\_{\text{2(g)}} \rightleftharpoons \textsf{2O}\_{\text{3(g)}} \tag{11}$$

The metastable nature of ozone makes it transient, having the tendency of always converting to oxygen. It is a vigorous oxidizing agent, always reacting to give up oxygen gas

$$\rm{S^{2-}} + \rm{2O}\_{3(g)} \rightarrow \rm{SO}\_{4}^{2-} \rm{}\_{\text{(aq)}} + \rm{O}\_{2(g)} \tag{12}$$

At an altitude of about 25 km above the earth surface, dioxygen can be split apart by ultraviolet light radiation coming from the sun. Some of these atoms react with other oxygen molecules forming the ozone layer

$$\rm O\_{2(g)} + O\_{(g)} \to O\_{3(g)}\tag{13}$$

This reaction is of extreme importance to the maintenance of balance here on earth. This is because the ozone has the ability to absorb dangerous ultraviolet radiation from the sun, thus preventing it from reaching the earth surface. This radiation is of high energy, therefore, of short wavelength. If too much of this radiation reaches the earth, the energy balance will be upset leading to increase greenhouse effect and global atmospheric temperature.

Secondly, exposure to increase ultraviolet radiation will lead to cell mutation of living tissues. The consequences of this will increase in skin cancer incidence in particular.

### **4.8. Chemical properties of oxygen**

### *4.8.1. Oxides*

**4.6. Sources of oxygen**

**4.7. Ozone**

is set up.

Fractional distillation of air is the main source of obtaining oxygen. Air is forced under pressure through nozzles. The compressed air is allowed to expand into a region of lower pressure, which cools the air. The air is cooled further in expansion tubes until it condenses into liquid. The liquid air is a mixture of nitrogen, oxygen, and rare gases and is separated by allowing an increase in temperature of the medium. The other gases boil more easily than oxygen, so they evaporate leaving oxygen. Liquid oxygen, which is pale blue in color and

Oxygen extracted this way is used to aid respiration medically, for breathing by divers and

current on oxygen, concentration of about 10% realized this way. Ozone is blue in color like oxygen but diamagnetic. Pure ozone is a deep blue explosive liquid, which is obtained

action of ultraviolet light radiation on oxygen. Ozone located at about the altitude of 25 km is

Ozone is chemically found to be very endothermic and decomposes only slowly at 250°C in

Ozone is triangular in shape with equal bond length of about 128 nm. The degree of single and double bonds formed by ozone is same. Each oxygen atom of the ozone has six valence electrons. Describing this with valence bond theory, each bond will involve a set of resonance hybrid in which one of the bonds is a double bond and the other is a coordinate bond. Moreover, the real structure does not swap between the resonance frames; rather, each bond

In the laboratory, ozone is made by passing oxygen through an electric field. An equilibrium

The metastable nature of ozone makes it transient, having the tendency of always converting

At an altitude of about 25 km above the earth surface, dioxygen can be split apart by ultraviolet light radiation coming from the sun. Some of these atoms react with other oxygen mol-

2−

to oxygen. It is a vigorous oxidizing agent, always reacting to give up oxygen gas

S2<sup>−</sup> + 2O3(g) → SO4

). Ozone (O3

) is formed by the action of electric

mixture. In the atmosphere, ozone comes about by the

**<sup>2</sup>** O, ∆H = −142 kJmol**<sup>−</sup>**<sup>1</sup> (10)

3O2(g) ⇌ 2O3(g) (11)

(aq) + O2(g) (12)

strongly paramagnetic, is stored under pressure or in insulated containers.

astronauts, in oxy-acetylene welding and rocket fuels, etc.

112 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

–O3

responsible for preventing excess ultraviolet light from reaching on the earth.

partly shows a nature of a double bond and partly that of a single bond.

An allotrope of oxygen after dioxygen (O2

the presence of catalyst or ultraviolet (UV) light.

O3 ⇌ \_\_**<sup>3</sup>**

ecules forming the ozone layer

by fractional liquefaction of O2

The ability of oxygen molecule to combine with both metals and nonmetals to form oxides is its most outstanding property. Oxides are of four different types: neutral, basic, acidic, and amphoteric. Oxides that show both basic and acidic behavior are called amphoteric oxides. Few oxides are neutral: nitrous oxide and carbon monoxide are examples. Group I and II oxides are examples of basic oxides. Oxides of nonmetals are mainly acidic in nature. Aluminum oxide is an example of amphoteric oxide.

It shows basic properties by reacting with hydrogen ions

$$\rm Al\_2O\_{3(s)} + 6H^+\_{\text{(aq)}} \to 2Al^{3+} \_{\text{(aq)}} + 3H\_2O\_{(l)} \tag{14}$$

Additionally, it shows acidic property when dissolved in alkali

$$\mathrm{Al}\_2\mathrm{O}\_{3(s)} + \mathrm{6OH}^- + 3\mathrm{H}\_2\mathrm{O}\_{(l)} \to 2\mathrm{Al(OH)}\_6^{3-} \tag{15}$$

Formulae of some oxides in the periodic table:


Not all the oxides are shown.

### *4.8.2. Peroxides*

The most important peroxide of oxygen is hydrogen peroxide, H2 O2 . Hydrogen peroxide is a colorless liquid with a boiling point of 152.1 °C. It is similar to water in many of its properties and forms hydrogen bonding too, and it is 40% denser than water. It has high dielectric constant and so used as ionizing solvent, but its utility in this capacity is limited by its strong oxidizing nature, which makes it readily decompose in the presence of many heavy-metal ions as given in the equation below.

$$\rm{2H\_2O\_2} \rightarrow 2\rm{H\_2O} + \rm{O\_{2^\circ}} \,\rm{\Delta H=-99 \, K\text{J}} \,\rm{mol^{-1}} \tag{16}$$

manufacture of paints and pigments, electrolytes for heavy duty batteries, and laboratory reagents. Sulfuric acid has a dynamic chemistry. In dilute solution, it behaves as a typically strong acid. When concentrated, it behaves both like an oxidizing and a dehydrating agent. It

O(l) → HSO4

Only the first dissociation is complete; the second is partial. When it is diluted, it shows prop-

The acid shows oxidizing property in concentrated form. For example, sulfuric acid cannot be used to prepare hydrogen bromide from sodium bromide. This is because it can oxidize the

This oxidizing property is a feature of sulfate ion. Since the ion has a high oxidation state of

Concentrated sulfuric acid will remove water from various organic compounds as can be noted when few drops of it are added to sugar (glucose). The sugar suddenly becomes very

Hydrogen sulfide is an important hydride of sulfur. It is a very poisonous gas, and when inhaled for some time, it can be fatal. The gas is made by mixing hydrochloric acid with a

A useful property of hydrogen sulfide is that it releases sulfide ions when dissolved in water.

S(g) + 3O<sup>2</sup> → 2H2

SO4(l) → Br2(l) + SO<sup>2</sup> + 2H<sup>2</sup>

O(l) → SO4

<sup>−</sup> + H<sup>3</sup> O<sup>+</sup>

Periodic Trends among Interstellar Molecular Species: The Case of Oxygen- and Sulfur…

2−

SO4(aq) → ZnSO4(aq) + H2(g) (19)

(aq) (17)

O(l) (20)

O(g) + 2SO2(g) (21)

(aq) (18)

http://dx.doi.org/10.5772/intechopen.80884

115

In water, sulfuric acid behaves as a strong acid. It dissociates in to two stages

HSO4−(aq) + H<sup>2</sup>

SO4(aq) + H<sup>2</sup>

erties of an acid. It will evolve hydrogen when it reacts with metals

Zn(s) + H<sup>2</sup>

2HBr(g) + H<sup>2</sup>

+6, it makes it to take electrons to revert to a lower oxidation state.

Unlike water, hydrogen sulfide will burn in air with a pale blue flame.

2H2

is also a sulfonating agent in organic chemistry.

H2

**Sulfuric acid as a strong acid:**

**Sulfuric acid as oxidizing agent:**

hydrogen bromide produced.

**Sulfuric acid as a dehydrating agent:**

metal sulfide, often iron (II) sulfide.

**Sulfur hydrides:**

hot and frothy, leaving a black mass of carbon.

### **4.9. Sulfur**

Sulfur is the second member of the oxygen group in the periodic table. Sulfur has more allotropic forms than any other elements. These different forms of allotropes are to the extent to which sulfur is polymerized and the crystal structure adopted. The α or rhombic and β or monoclinic sulfur are the two most common ones. Sulfur is not a gas unlike oxygen and has a significantly lower electronegativity. They only react with group one element to form ionic compounds. In many sulfur compounds, the d-orbital is used in bonding and these bonds appear shorter than expected, which suggest a double bond character. Sulfur can make up to six covalent bonds making use of its s-, p-, and d-orbitals. Sulfates and hexafluorides are examples of this instance.

### **Physical property of sulfur:**


#### **Uses of sulfur:**

Sulfur has several purposes of uses; it is mainly used as sulfuric acid. It is also used in fertilizer, explosives, dyes, detergents, polymers, and in processing of many other chemicals.

#### **Extraction of sulfur:**

Sulfur is found in many minerals, mostly in combination with copper, mercury, lead metals. Sulfur is obtained as the byproduct of the extraction of their ore. Sulfur is also directly extracted from the ground using a method called the *Frasch* process. Sulfur in the form of hydrogen sulfide is also obtained from oil and natural gas refineries.

#### **Sulfuric acid:**

This is used in the manufacturing of superphosphate fertilizer, ammonium sulfate fertilizer, detergents, paper, rayon, polymer, and processing of metal ores. It is also used in the manufacture of paints and pigments, electrolytes for heavy duty batteries, and laboratory reagents. Sulfuric acid has a dynamic chemistry. In dilute solution, it behaves as a typically strong acid. When concentrated, it behaves both like an oxidizing and a dehydrating agent. It is also a sulfonating agent in organic chemistry.

### **Sulfuric acid as a strong acid:**

and forms hydrogen bonding too, and it is 40% denser than water. It has high dielectric constant and so used as ionizing solvent, but its utility in this capacity is limited by its strong oxidizing nature, which makes it readily decompose in the presence of many heavy-metal

Sulfur is the second member of the oxygen group in the periodic table. Sulfur has more allotropic forms than any other elements. These different forms of allotropes are to the extent to which sulfur is polymerized and the crystal structure adopted. The α or rhombic and β or monoclinic sulfur are the two most common ones. Sulfur is not a gas unlike oxygen and has a significantly lower electronegativity. They only react with group one element to form ionic compounds. In many sulfur compounds, the d-orbital is used in bonding and these bonds appear shorter than expected, which suggest a double bond character. Sulfur can make up to six covalent bonds making use of its s-, p-, and d-orbitals. Sulfates and hexafluorides are examples of this instance.

Sulfur has several purposes of uses; it is mainly used as sulfuric acid. It is also used in fertilizer, explosives, dyes, detergents, polymers, and in processing of many other chemicals.

Sulfur is found in many minerals, mostly in combination with copper, mercury, lead metals. Sulfur is obtained as the byproduct of the extraction of their ore. Sulfur is also directly extracted from the ground using a method called the *Frasch* process. Sulfur in the form of

This is used in the manufacturing of superphosphate fertilizer, ammonium sulfate fertilizer, detergents, paper, rayon, polymer, and processing of metal ores. It is also used in the

hydrogen sulfide is also obtained from oil and natural gas refineries.

, ∆H = −99 KJmol−<sup>1</sup> (16)

3p4

O + O<sup>2</sup>

ions as given in the equation below.

**Physical property of sulfur:**

**Uses of sulfur:**

I.E. (KJmol−<sup>1</sup>

**Sulfuric acid:**

**Extraction of sulfur:**

**4.9. Sulfur**

2H2

114 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

O2 → 2H2

**Symbol Sulfur** Electron structure (Ne) 3S2

) 1000

Electronegativity 2.5

Melting point (°C) 114.5 Boiling point (°C) 444.5 Atomic radius (pm) 104 Principal oxidation number −2, +4, +6 In water, sulfuric acid behaves as a strong acid. It dissociates in to two stages

$$\rm H\_2SO\_{4(aq)} + H\_2O\_{(l)} \to HSO\_4^- + H\_3O^+\_{(aq)}\tag{17}$$

$$\text{HSO}\_{4-\text{(aq)}} + \text{H}\_2\text{O}\_{\text{(l)}} \rightarrow \text{SO}\_4^{2-} \tag{18}$$

Only the first dissociation is complete; the second is partial. When it is diluted, it shows properties of an acid. It will evolve hydrogen when it reacts with metals

$$\text{Zn}\_{\text{(s)}} + \text{H}\_2\text{SO}\_{4\text{(aq)}} \to \text{ZnSO}\_{4\text{(aq)}} + \text{H}\_{2\text{(g)}}\tag{19}$$

### **Sulfuric acid as oxidizing agent:**

The acid shows oxidizing property in concentrated form. For example, sulfuric acid cannot be used to prepare hydrogen bromide from sodium bromide. This is because it can oxidize the hydrogen bromide produced.

$$2\text{HBr}\_{\text{(g)}} + \text{H}\_{2}\text{SO}\_{\text{4(l)}} \rightarrow \text{Br}\_{2\text{(l)}} + \text{SO}\_{2} + 2\text{H}\_{2}\text{O}\_{\text{(l)}}\tag{20}$$

This oxidizing property is a feature of sulfate ion. Since the ion has a high oxidation state of +6, it makes it to take electrons to revert to a lower oxidation state.

### **Sulfuric acid as a dehydrating agent:**

Concentrated sulfuric acid will remove water from various organic compounds as can be noted when few drops of it are added to sugar (glucose). The sugar suddenly becomes very hot and frothy, leaving a black mass of carbon.

### **Sulfur hydrides:**

Hydrogen sulfide is an important hydride of sulfur. It is a very poisonous gas, and when inhaled for some time, it can be fatal. The gas is made by mixing hydrochloric acid with a metal sulfide, often iron (II) sulfide.

Unlike water, hydrogen sulfide will burn in air with a pale blue flame.

$$2\text{H}\_2\text{S}\_{\text{(g)}} + 3\text{O}\_2 \rightarrow 2\text{H}\_2\text{O}\_{\text{(g)}} + 2\text{SO}\_{2(g)}\tag{21}$$

A useful property of hydrogen sulfide is that it releases sulfide ions when dissolved in water.

### **5. Summary**

The first part (Sections 1 to 4) of this chapter, which is based on research, discusses the nonterrestrial chemistry of oxygen, sulfur, and their compounds in the interstellar medium, while the second part of the chapter (Section 5) discusses about the basic inorganic chemistry of oxygen, sulfur, and oxygen. Both parts of this chapter point out the importance of these elements and their compounds in both terrestrial and nonterrestrial environments. Also, the importance of chemistry in these environments cannot be overemphasized.

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### **6. Conclusions**

The deviation of the observed S/O abundance from the rule that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio and the possibility of detecting other analogues of the known S- and O-containing species have been examined in this study. The effect of hydrogen bonding on the surface of the interstellar dust grains where these molecules are believed to be formed plays a vital role in the observed S/O abundance ratio. From the binding energy of these species with the water molecule on the surface of the dust grains, the more the molecules are strongly bonded to the surface of the dust grains, the more their abundances are reduced. As the ratio of the binding energy of S- and O-species (B. E. of S/O) with water approaches unity, the S/O abundance ratio approaches cosmic S/O ratio. When this ratio is less than one, the observed S/O abundance ratio becomes much higher than the cosmic S/O ratio and vice versa except for the species that are major components of the interstellar ice. With respect to the detectability of the unknown analogues of these species, every known O-species is an indication of the presence and detectability of the S-analogue. This has been shown to be true in many cases where the S-analogues of known O-species are successfully observed, following the detection of the O-analogues. That these S-containing species are less bonded to the surface of the interstellar dust grains as compared to their O-analogues firmly support the high abundances and the detectability of these species. For the known S-species whose O-analogues are yet to be observed, the O-analogues are not only present in detectable abundance, it can be said to have even been overdue for astronomical detection since the O-species without any exception are more abundant than their S-analogues and as such they could be detected with less difficulty as compared to their S-analogues that are already known. The second part of this chapter discusses the basic inorganic chemistry of hydrogen, oxygen, and sulfur.

### **Author details**

Etim Emmanuel1 \*, Lawal Usman<sup>1</sup> , Khanal Govinda2 and Mbakara Idaresit3


### **References**

**5. Summary**

**6. Conclusions**

**Author details**

Etim Emmanuel1

\*, Lawal Usman<sup>1</sup>

\*Address all correspondence to: emmaetim@gmail.com

3 Department of Chemistry, University of Ibadan, Oyo State, Nigeria

The first part (Sections 1 to 4) of this chapter, which is based on research, discusses the nonterrestrial chemistry of oxygen, sulfur, and their compounds in the interstellar medium, while the second part of the chapter (Section 5) discusses about the basic inorganic chemistry of oxygen, sulfur, and oxygen. Both parts of this chapter point out the importance of these elements and their compounds in both terrestrial and nonterrestrial environments. Also, the

The deviation of the observed S/O abundance from the rule that the ratio of an interstellar sulfur molecule to its oxygen analogue is close to the cosmic S/O ratio and the possibility of detecting other analogues of the known S- and O-containing species have been examined in this study. The effect of hydrogen bonding on the surface of the interstellar dust grains where these molecules are believed to be formed plays a vital role in the observed S/O abundance ratio. From the binding energy of these species with the water molecule on the surface of the dust grains, the more the molecules are strongly bonded to the surface of the dust grains, the more their abundances are reduced. As the ratio of the binding energy of S- and O-species (B. E. of S/O) with water approaches unity, the S/O abundance ratio approaches cosmic S/O ratio. When this ratio is less than one, the observed S/O abundance ratio becomes much higher than the cosmic S/O ratio and vice versa except for the species that are major components of the interstellar ice. With respect to the detectability of the unknown analogues of these species, every known O-species is an indication of the presence and detectability of the S-analogue. This has been shown to be true in many cases where the S-analogues of known O-species are successfully observed, following the detection of the O-analogues. That these S-containing species are less bonded to the surface of the interstellar dust grains as compared to their O-analogues firmly support the high abundances and the detectability of these species. For the known S-species whose O-analogues are yet to be observed, the O-analogues are not only present in detectable abundance, it can be said to have even been overdue for astronomical detection since the O-species without any exception are more abundant than their S-analogues and as such they could be detected with less difficulty as compared to their S-analogues that are already known. The second part of this

importance of chemistry in these environments cannot be overemphasized.

116 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

chapter discusses the basic inorganic chemistry of hydrogen, oxygen, and sulfur.

, Khanal Govinda2

1 Department of Chemical Sciences, Federal University Wukari, Taraba State, Nigeria

2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India

and Mbakara Idaresit3


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**Chapter 7**

Provisional chapter

**Mechanism of Interactions of Zinc(II) and Copper(II)**

DOI: 10.5772/intechopen.79472

Over the past few decades, transition metal complexes have attracted considerable attention in medicinal inorganic chemistry, especially as synthetic metallonucleases and metalbased anticancer drugs that are able to bind to DNA under physiological conditions. The use of metal-based drugs presents the most important strategy in the development of new anticancer and antimicrobial agents. Negative side effects during treatment (such as vomiting, resistance, nephrotoxicity, ototoxicity, neurotoxicity and cardiotoxicity) prompted researchers to design new classes of DNA and protein targeting metal-based anticancer agents with potential in vitro selectivity and less toxicity. Knowledge of mechanism of the interaction zinc(II) and copper (II) ions with biomolecules and other relevant ligands is essential for understanding the cellular biology of delivery complexes to DNA and proteins. Results obtained from investigations provide useful information for the future design of potential zinc- and copper-based anticancer drugs. Different mechanism of interactions with selected biomolecules compared to platinum-based drugs has been observed.

Keywords: transition metal complexes, kinetics, mechanism of interactions, metal-based

The aim of this chapter is to present fundamental chemical properties and new investigations of coordination compounds of some transition metal ions with an overview of medicinal applications. Transition metals appear in almost every facet of our day-to-day life, from industrial uses such as the manufacture of construction and building materials, tools, vehicles, up to cosmetics, paints and fertilizers. Their reactions are in general important in many

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Mechanism of Interactions of Zinc(II) and Copper(II)

**Complexes with Small Biomolecules**

Complexes with Small Biomolecules

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

drugs, biomolecules, antitumour activity

http://dx.doi.org/10.5772/intechopen.79472

Tanja Soldatović

Abstract

1. Introduction

Tanja Soldatović


#### **Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules** Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

DOI: 10.5772/intechopen.79472

Tanja Soldatović Tanja Soldatović

[85] Snyder LE, Buhl D, Schwartz PR, et al. Radio detection of interstellar dimethyl ether. The

[86] Hollis JM, Lovas FJ, Remijan AJ, et al. Detection of acetamide (CH3CONH2): The largest interstellar molecule with a peptide bond. The Astrophysical Journal. 2006;**643**:L25 [87] Snyder LE, Lovas FJ, Mehnringer DM, et al. Confirmation of nterstellar acetone. The

[88] Belloche A, Garrod RT, Müller HSP, et al. Increased complexity in interstellar chemistry: Detection and chemical modeling of ethyl formate and n-propyl cyanide in Sagittarius

[89] Tercero B, Kleiner I, Cernicharo J, et al. Discovery of methyl acetate and gauche ethyl

[90] Fuchs GW, Fuchs U, Giesen TF, Wyrowski F. Trans-ethyl methyl ether in space a new look at a complex molecule in selected hot core regions. Astronomy & Astrophysics.

[91] Snyder LE, Hollis JM, Ulich BL, et al. Radio detection of interstellar sulfur dioxide. The

[92] Turner BE. Detection of interstellar SO(+)—A diagnostic of dissociative shock chemistry.

[93] Agúndez M, Cernicharo J, Guélin M. New molecules in IRC +10216: Confirmation

[94] Etim EE, Gorai P, Das A, Chakrabarti SK, Arunan E. Interstellar hydrogen bonding. Advances in Space Research. 2018;61(11):2870-2880. DOI: 10.1016/j.asr.2018.03.003 [95] Vidal THG, Wakelam V. A new look at Sulphur chemistry in hot cores and corinos.

[96] Fuente A, Goicoechea J-R, Pety J, et al. First detection of interstellar S2H. The Astrophysical

[97] Vidal THG, Loison J-C, Jaziri AY, et al. On the reservoir of Sulphur in dark clouds: Chemistry and elemental abundance reconciled. Monthly Notices of the Royal Astro-

CN\*\*. Astronomy &

S and tentative identification of MgCCH, NCCP, and SiH<sup>3</sup>

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Astrophysical Journal. 2002;**578**:245

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nomical Society. 2017;**469**:435

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2005;**444**:521

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formate in orion\*. The Astrophysical Journal. 2013;**770**:L13

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79472

### Abstract

Over the past few decades, transition metal complexes have attracted considerable attention in medicinal inorganic chemistry, especially as synthetic metallonucleases and metalbased anticancer drugs that are able to bind to DNA under physiological conditions. The use of metal-based drugs presents the most important strategy in the development of new anticancer and antimicrobial agents. Negative side effects during treatment (such as vomiting, resistance, nephrotoxicity, ototoxicity, neurotoxicity and cardiotoxicity) prompted researchers to design new classes of DNA and protein targeting metal-based anticancer agents with potential in vitro selectivity and less toxicity. Knowledge of mechanism of the interaction zinc(II) and copper (II) ions with biomolecules and other relevant ligands is essential for understanding the cellular biology of delivery complexes to DNA and proteins. Results obtained from investigations provide useful information for the future design of potential zinc- and copper-based anticancer drugs. Different mechanism of interactions with selected biomolecules compared to platinum-based drugs has been observed.

Keywords: transition metal complexes, kinetics, mechanism of interactions, metal-based drugs, biomolecules, antitumour activity

### 1. Introduction

The aim of this chapter is to present fundamental chemical properties and new investigations of coordination compounds of some transition metal ions with an overview of medicinal applications. Transition metals appear in almost every facet of our day-to-day life, from industrial uses such as the manufacture of construction and building materials, tools, vehicles, up to cosmetics, paints and fertilizers. Their reactions are in general important in many

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

technical processes such as catalysis, materials synthesis, photochemistry, as well as, in biology and medicine.

It is known that metal ions play an important role in the biological and biomedical processes. Namely, many processes such as breathing, metabolism, photosynthesis, growth, reproduction, muscle contraction cannot imagine without the presence of some metal ions. It is currently believed that about 24 elements are essential for the life of mammals, which are: H, C, N, O, F, Na, Mg, Si, P, S, Cl, K, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Sn and I.

The field of inorganic coordination chemistry, among other thing, is concerned with the study of the use of compounds of essential and non-essential elements in medicine, as well as, of the interaction of given compounds with the present biomolecules within the organism. Now many inorganic coordination compounds are widely used in medicine for the treatment of many diseases, including various cancers, Alzheimer's disease, diabetes, rheumatoid arthritis and others. In this chapter, among other things, we are focused on coordination compounds zinc(II) and copper(II) and on investigation of the mechanism of interactions with biologically relevant molecules.

### 2. Transition metal ions chemistry

### 2.1. Lewis acid base theory

Although, in this chapter, we mainly discuss the coordination compounds of transition metal ions, it is very important to explain some of the basic characteristics of complex compounds such as are definition of Lewis acids and bases.

In practice, the character of the metal-oxygen interaction varies with the nature of the metal

Figure 1. Left: the first hydration shell of an Na<sup>+</sup> ion; ion-dipole interactions between the Na<sup>+</sup> ion and the H2O molecules;

Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

http://dx.doi.org/10.5772/intechopen.79472

125

In concentrated solutions, the plane of the water molecule makes an angle of up to 50 degree with the Mz+O axis (Figure 2) implying interaction of the cation with a lone pair of electrons rather than an ion-dipole interaction, it suggests that the metal-oxygen interaction involves the

Metal ions in aqueous solution behave as Lewis acids. The positive charge on the metal ion draws electron density from the O-H bond in the water. This increases the bond's polarity making it easier to break. When the O-H bond breaks, an aqueous proton is released producing an acidic solution. Hydrolysis refers to the reversible loss of H<sup>+</sup> from an aqua species.

Transition metal ions can act as Brønsted acids by loss of H+ from a coordinated water

cannot be Brønsted acids by themselves. Water molecules covalently bound to one of these metal ions are more acidic than normal, the H atoms carry significant positive charge. Thus,

, Mg2+, Al3+, Fe3+ and Ti3+ possess high charge densities,

z+, where z is

use of an oxygen lone pair. Metal cations are equated with the formula [M(H2O)n]

ion and relevant to this is the electroneutrality principle (Figure 1).

Figure 2. The plane of the water molecule in the direction Mz+O axis.

1, 2 or 3, and they tend to hydrolyze [1].

Right: bonding of metal ions and H2O molecules.

molecule. Small cations such as Li+

reactions such as the following occur.

2.3. Transition metal ions as Brønsted acids

A Lewis acid is an electron acceptor and a Lewis base is an electron donor. In a coordination complex, the central metal ions act as a Lewis acid and are coordinated (bonded) by one or more molecules or ions (ligands) which act as Lewis bases. Formed coordinated bonds between central atom or ion with ligands have covalent character and are known under name coordinate covalent bond or simple coordinate bond. Atoms in the ligands that are directly bonded to the central atom or ion are donor atoms.

### 2.2. Hydration or hydrolysis of metal ions?

When a metal salt is dissolved in water, the ionic bond is interrupted, the cations and anions are hydrated. For example, when NaCl is dissolved in water, the inner hydration shell around Na<sup>+</sup> is formed. The Na+ O interaction can be described in terms of an ion-dipole interaction, while the solvation of the anion can be described in terms of the formation of hydrogen bonds between Cl and H atoms of surrounding H2O molecules (Figure 1).

Hydration is solvation when the solvent is water. If the metal-oxygen bond possesses covalent character, there is also an ionic contribution to the bonding interaction. Each O atom donates a pair of electrons to the metal Mz+ ion, and each H2O molecule acts as a Lewis base while the metal ion acts as a Lewis acid. The M-O interaction is covalent, in contrast to the case for Na+ . Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules http://dx.doi.org/10.5772/intechopen.79472 125

Figure 1. Left: the first hydration shell of an Na<sup>+</sup> ion; ion-dipole interactions between the Na<sup>+</sup> ion and the H2O molecules; Right: bonding of metal ions and H2O molecules.

Figure 2. The plane of the water molecule in the direction Mz+O axis.

technical processes such as catalysis, materials synthesis, photochemistry, as well as, in

It is known that metal ions play an important role in the biological and biomedical processes. Namely, many processes such as breathing, metabolism, photosynthesis, growth, reproduction, muscle contraction cannot imagine without the presence of some metal ions. It is currently believed that about 24 elements are essential for the life of mammals, which are: H, C, N,

The field of inorganic coordination chemistry, among other thing, is concerned with the study of the use of compounds of essential and non-essential elements in medicine, as well as, of the interaction of given compounds with the present biomolecules within the organism. Now many inorganic coordination compounds are widely used in medicine for the treatment of many diseases, including various cancers, Alzheimer's disease, diabetes, rheumatoid arthritis and others. In this chapter, among other things, we are focused on coordination compounds zinc(II) and copper(II) and on investigation of the mechanism of interactions with biologically

Although, in this chapter, we mainly discuss the coordination compounds of transition metal ions, it is very important to explain some of the basic characteristics of complex compounds

A Lewis acid is an electron acceptor and a Lewis base is an electron donor. In a coordination complex, the central metal ions act as a Lewis acid and are coordinated (bonded) by one or more molecules or ions (ligands) which act as Lewis bases. Formed coordinated bonds between central atom or ion with ligands have covalent character and are known under name coordinate covalent bond or simple coordinate bond. Atoms in the ligands that are directly bonded

When a metal salt is dissolved in water, the ionic bond is interrupted, the cations and anions are hydrated. For example, when NaCl is dissolved in water, the inner hydration shell around Na<sup>+</sup> is formed. The Na+ O interaction can be described in terms of an ion-dipole interaction, while the solvation of the anion can be described in terms of the formation of hydrogen bonds

Hydration is solvation when the solvent is water. If the metal-oxygen bond possesses covalent character, there is also an ionic contribution to the bonding interaction. Each O atom donates a pair of electrons to the metal Mz+ ion, and each H2O molecule acts as a Lewis base while the metal ion acts as a Lewis acid. The M-O interaction is covalent, in contrast to the case for Na+

.

between Cl and H atoms of surrounding H2O molecules (Figure 1).

O, F, Na, Mg, Si, P, S, Cl, K, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Sn and I.

124 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

biology and medicine.

relevant molecules.

2.1. Lewis acid base theory

2. Transition metal ions chemistry

such as are definition of Lewis acids and bases.

to the central atom or ion are donor atoms.

2.2. Hydration or hydrolysis of metal ions?

In practice, the character of the metal-oxygen interaction varies with the nature of the metal ion and relevant to this is the electroneutrality principle (Figure 1).

In concentrated solutions, the plane of the water molecule makes an angle of up to 50 degree with the Mz+O axis (Figure 2) implying interaction of the cation with a lone pair of electrons rather than an ion-dipole interaction, it suggests that the metal-oxygen interaction involves the use of an oxygen lone pair. Metal cations are equated with the formula [M(H2O)n] z+, where z is 1, 2 or 3, and they tend to hydrolyze [1].

#### 2.3. Transition metal ions as Brønsted acids

Metal ions in aqueous solution behave as Lewis acids. The positive charge on the metal ion draws electron density from the O-H bond in the water. This increases the bond's polarity making it easier to break. When the O-H bond breaks, an aqueous proton is released producing an acidic solution. Hydrolysis refers to the reversible loss of H<sup>+</sup> from an aqua species.

Transition metal ions can act as Brønsted acids by loss of H+ from a coordinated water molecule. Small cations such as Li+ , Mg2+, Al3+, Fe3+ and Ti3+ possess high charge densities, cannot be Brønsted acids by themselves. Water molecules covalently bound to one of these metal ions are more acidic than normal, the H atoms carry significant positive charge. Thus, reactions such as the following occur.

$$\begin{array}{rcl} \text{[Fe(H}\_{2}\text{O)}\_{5}\text{(OH)]}^{2+} + \text{H}\_{2}\text{O} \xrightarrow{\text{}} & \text{[Fe(H}\_{2}\text{O)}\_{4}\text{(OH)}\_{2}\text{]}^{+} + \text{H}\_{3}\text{O}^{+}\\ \text{pK}\_{\text{eq}}\text{([Fe(H}\_{2}\text{O)}\_{6}\text{]}^{3+}) \sim \text{pK}\_{\text{a}}\text{(HNO}\_{2}\text{)} \end{array}$$

The characteristic colour of the [Fe(H2O)6] 3+ ion is purple, but aqueous solutions appear yellow due to the formation of the hydroxo species [Fe(H2O)5(OH)]2+ and [Fe(H2O)4(OH)2] + .

The equilibrium constant Keq for the hydrolysis of a hydrated cation is analogous to the Ka for the ionization of a weak acid. Generally, hydrolysis constants for cations are signed as �log Ka. These hydrolysis constants are averages of different experimental measurements. If we compare the value of constant for previous reaction with Ka of weak acids, it can be seen that pKeq of [Fe(H2O6)]3+ correspond to pKa of weak nitrous acid.

#### 2.4. Stability constants of coordination complexes

Metal ions in aqueous solution are hydrated; the aqua species may be accounted as Mz+(aq) where this often represents the hexaaqua ion [M(H2O)6] n+. Addition of a neutral ligand L to the solution leads the formation of a series of complexes [M(H2O)5L]n+, [M(H2O)4L2] n+…[ML6] n+. The stepwise displacements of coordinated H2O by L are represented by Eqs. (1) and (2).

$$\begin{aligned} \text{[M(H\_2O)\_6]}^{x+} + \text{L} &\xleftarrow{\text{K}\_1} [\text{M(H\_2O)\_5L}]^{x+} + \text{H}\_2\text{O} \\ \text{K}\_1 &= \frac{[\text{M(H\_2O)\_5L}]^{x+}}{[\text{M(H\_2O)\_6}]^{x+}[\text{L}]} \end{aligned} \tag{1}$$

2.5. Hard and soft acid base principle

Al3+, Ga3+, In3+, Sc3+, Cr3+, Fe3+, Co3+, Y3+, Th4+, Pu4+,

+

F, Cl, H2O, ROH, R2O, [OH], [RO], [RCO2]

<sup>3</sup>, [SO4]

, [PO4]

example, Au<sup>+</sup>

Li+ , Na<sup>+</sup> , K+ , Rb+

[CO3]

NH3, RNH2

behaviour.

Ti4+, Zr4+, [VO]2+, [VO2]

<sup>2</sup>, [NO3]

overlap between them.

will be favourable [1].

3.1. Medicinal inorganic chemistry

Based on acceptor properties of metal ions towards ligands (i.e. Lewis acid-Lewis base interactions), two classes of metal ion can be identified, although the distinction between them is not clear-cut. The terms "hard" and "soft" acids arise from a polarizabilities of the metal ions. Hard acids are typically either small monocations with a relatively high charge density or are highly charged, again with a high charge density. These ions are not very polarizable and show a preference for donor atoms that are also not very polarizable, for example, F. Such ligands are called hard bases. Soft acids tend to be large monocations with a low charge density, for

Pb2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Os2+, Ru3+,

(N-bound), ArNH2,

, [SO3] 2

Table 1. Selected hard and soft metal centres (Lewis acids) and ligands (Lewis bases) and those that exhibit intermediate

, py, [SCN]

Zero oxidation state metal centres, Tl+

R3As, R3Sb, alkenes, arene

, H, R, [CN] (C-bound), CO (C-bound), RNC, RSH, R2S, [RS], [SCN] (S-bound), R3P,

http://dx.doi.org/10.5772/intechopen.79472

2+, Hg2+, Cd2+, Pd2+, Pt2+, Ru2+ Tl3+

Au<sup>+</sup> , [Hg2]

Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

I

, Cu<sup>+</sup> , Ag<sup>+</sup> , 127

Rh3+, Ir2+

Br, [N3]

[NO2]

atoms that are also highly polarizable, for example, I. Such ligands are called soft bases [1]. Hard acids (hard metal cations) form more stable complexes with hard bases (hard ligands), while soft acids (soft metal cations) show a preference for soft bases (soft ligands). The list of

The hard and soft acid base (HSAB) principle is qualitatively useful, the hard-hard or soft-soft matching of acid and base represents a stabilization that is additional to other factors that contribute to the strength of the bonds between donor and acceptor. These factors include the sizes of the cation and donor atom, their charges, their electronegativities and the orbital

Complex formation usually involves ligand substitution. If we suppose that Mz+ is a hard acid. It is already associated with hard H2O ligands, and hard-hard interaction is a favourable. If L is a soft base, ligand substitution will not be favourable. If Mz+ is a soft acid, ligand substitution

3. Medicinal application of inorganic complexes (metal-based drugs)

are now used in medicine for the treatment of numerous diseases.

Medicinal inorganic chemistry is a part of bioinorganic chemistry that occupies a significant place in the field of therapeutic and diagnostic medicine. Inorganic coordination compounds

hard and soft acids and bases with included intermediates is shown in Table 1.

Hard (acids) Intermediate (acids) Soft (acids)

Hard (bases) Intermediate (bases) Soft (bases)

, [ox]<sup>2</sup>,

,

, Be2+, Mg2+, Ca2+, Sr2+, Sn2+, Mn2+,

<sup>2</sup>, [ClO4]

, and are very polarizable. They prefer to form coordinate bonds with donor

$$\begin{aligned} \text{[M(H\_2O)L\_5]}^{\text{x+}} + & \quad \text{L} \xleftarrow{\text{K}\_6} \quad \text{[ML\_6]}^{\text{x+}} + & \quad \text{H}\_2\text{O} \\ & \quad \text{K}\_6 = \frac{[\text{ML}\_6]^{\text{x+}}}{[\text{M(H\_2O)L\_5}]^{\text{x+}}[\text{L}]} \end{aligned} \tag{2}$$

In step-wise formation of complex [ML6] <sup>+</sup> from [M(H2O)6] z+, each displacement of a coordinated water molecule by ligand L has a characteristic stepwise stability constant, K1, K2, K3, K4, K5 or K6. Alternatively, we may consider the overall formation of [ML6] z+ (Eq. (3)).

$$\begin{array}{rcl} \text{[M(H\_2O)\_6]}^{\text{x+}} + & 6\text{L} \xrightarrow{\text{g}} & \text{[ML\_6]}^{\text{x+}} + & 6\text{H}\_2\text{O} \\\\ \text{\(\mathcal{H}\_6\)}^{\text{e}} & \begin{array}{rcl} \text{[ML\_6]}^{\text{x+}} \\ \text{[M(H\_2O)\_6]}^{\text{x+}} \text{[L]}^6 \end{array} \end{array} \tag{3}$$

The constant β<sup>6</sup> we call as cumulative stability constant. The connection between values of stepwise formation stability constant and overall stability constant is given by expression: β<sup>6</sup> = K1K2K3K4K5K6 or logβ<sup>6</sup> = logK1 + logK2 + logK3 + logK4 + logK5 + logK6. Determinations of stability constants can be made by polarographic or potentiometric measurements (if a suitable reversible electrode exists), by pH measurements (if the ligand is the conjugate base of a weak acid) or by ion-exchange, spectrophotometric (i.e. observation of electronic spectra and use of the Beer–Lambert Law), NMR spectroscopic or distribution methods [1].


Table 1. Selected hard and soft metal centres (Lewis acids) and ligands (Lewis bases) and those that exhibit intermediate behaviour.

### 2.5. Hard and soft acid base principle

The characteristic colour of the [Fe(H2O)6]

of [Fe(H2O6)]3+ correspond to pKa of weak nitrous acid.

126 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

where this often represents the hexaaqua ion [M(H2O)6]

2.4. Stability constants of coordination complexes

In step-wise formation of complex [ML6]

due to the formation of the hydroxo species [Fe(H2O)5(OH)]2+ and [Fe(H2O)4(OH)2]

The equilibrium constant Keq for the hydrolysis of a hydrated cation is analogous to the Ka for the ionization of a weak acid. Generally, hydrolysis constants for cations are signed as �log Ka. These hydrolysis constants are averages of different experimental measurements. If we compare the value of constant for previous reaction with Ka of weak acids, it can be seen that pKeq

Metal ions in aqueous solution are hydrated; the aqua species may be accounted as Mz+(aq)

<sup>+</sup> from [M(H2O)6]

nated water molecule by ligand L has a characteristic stepwise stability constant, K1, K2, K3, K4,

The constant β<sup>6</sup> we call as cumulative stability constant. The connection between values of stepwise formation stability constant and overall stability constant is given by expression: β<sup>6</sup> = K1K2K3K4K5K6 or logβ<sup>6</sup> = logK1 + logK2 + logK3 + logK4 + logK5 + logK6. Determinations of stability constants can be made by polarographic or potentiometric measurements (if a suitable reversible electrode exists), by pH measurements (if the ligand is the conjugate base of a weak acid) or by ion-exchange, spectrophotometric (i.e. observation of electronic spectra

and use of the Beer–Lambert Law), NMR spectroscopic or distribution methods [1].

K5 or K6. Alternatively, we may consider the overall formation of [ML6]

solution leads the formation of a series of complexes [M(H2O)5L]n+, [M(H2O)4L2]

The stepwise displacements of coordinated H2O by L are represented by Eqs. (1) and (2).

3+ ion is purple, but aqueous solutions appear yellow

n+. Addition of a neutral ligand L to the

z+, each displacement of a coordi-

z+ (Eq. (3)).

+ .

n+…[ML6]

n+.

ð1Þ

ð2Þ

ð3Þ

Based on acceptor properties of metal ions towards ligands (i.e. Lewis acid-Lewis base interactions), two classes of metal ion can be identified, although the distinction between them is not clear-cut. The terms "hard" and "soft" acids arise from a polarizabilities of the metal ions. Hard acids are typically either small monocations with a relatively high charge density or are highly charged, again with a high charge density. These ions are not very polarizable and show a preference for donor atoms that are also not very polarizable, for example, F. Such ligands are called hard bases. Soft acids tend to be large monocations with a low charge density, for example, Au<sup>+</sup> , and are very polarizable. They prefer to form coordinate bonds with donor atoms that are also highly polarizable, for example, I. Such ligands are called soft bases [1]. Hard acids (hard metal cations) form more stable complexes with hard bases (hard ligands), while soft acids (soft metal cations) show a preference for soft bases (soft ligands). The list of hard and soft acids and bases with included intermediates is shown in Table 1.

The hard and soft acid base (HSAB) principle is qualitatively useful, the hard-hard or soft-soft matching of acid and base represents a stabilization that is additional to other factors that contribute to the strength of the bonds between donor and acceptor. These factors include the sizes of the cation and donor atom, their charges, their electronegativities and the orbital overlap between them.

Complex formation usually involves ligand substitution. If we suppose that Mz+ is a hard acid. It is already associated with hard H2O ligands, and hard-hard interaction is a favourable. If L is a soft base, ligand substitution will not be favourable. If Mz+ is a soft acid, ligand substitution will be favourable [1].

### 3. Medicinal application of inorganic complexes (metal-based drugs)

### 3.1. Medicinal inorganic chemistry

Medicinal inorganic chemistry is a part of bioinorganic chemistry that occupies a significant place in the field of therapeutic and diagnostic medicine. Inorganic coordination compounds are now used in medicine for the treatment of numerous diseases.

Today, it is well known that some metal ions are required for normal functions of organism. Lack of zinc, iron, copper, ions and so on can induce disease. Some metal ions such as arsenic, cadmium, chromium, lead and mercury can induce toxicity in humans. Even essential metal ions can be toxic when present in excess. An important aspect of medicinal bioinorganic chemistry is ability to understand all this in the molecular level and treat diseases caused by inadequate metal ion function constitutes.

3. the oxidation state of platinum in the complexes is +2 or +4

4. nitrogen-donor ligands have to contain at least one NH bond.

Figure 3 presents some of platinum complexes that are in the medicinal use worldwide.

(BBR3464) and complexes containing a ligand with an asymmetric carbon atom [6].

Figure 3. Platinum antitumour complexes with adopted commercial names.

The mechanism of the antitumour effects of platinum complexes consists in their binding to DNA molecules, thereby preventing replication and transcription of DNA, that is, preventing the process of uncontrolled cell growth [7–9]. From the moment of injection of the drugs in the body to their binding to DNA molecules, a large number of secondary processes happen that

The second-generation platinum(II) antitumour complexes are carboplatin, oxaliplatin, nonplatinum, zenithplatin, enloplatin, NK121, CI-973 and others. Instead of labile chloride ligands, they contain bidentate ligands such as 1,1-cyclobutanedicarboxylate, glycolate, and complexes with 1,2-diaminocyclohexane as an inert ligand, while the labile ligands are sulphates, malonates and other ligands [5]. The second-generation complexes based on the cisplatin structure were developed to improve toxicity and/or expand the range of useful anticancer activity. The third-generation platinum antitumour complexes are octahedral platinum(IV) coordination compounds with general formula cis-[PtA2X2Y2], where two labile monodentate or one labile bidentate ligand is labeled as Y2. The platinum(IV) drugs are orally administrated to patients. In the presence of various biomolecules such as cysteine or ascorbic acid, the redaction to Pt(II) occurred by leaving the axial ligands Y2. In addition, this group includes new complexes with a trans-geometric structure, polynuclear platinum complexes

Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

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129

Medicinal inorganic chemistry is a multidisciplinary field combining elements of chemistry (synthesis and reactivity), pharmacology (pharmacokinetics and toxicology), biochemistry (targets, structure and conformational changes) and medicinal chemistry (therapeutics, pharmacodynamics and structure-activity relationships). The main focus of this field is to design of novel therapeutic and diagnostic agents and to investigate the mechanism of medicinal action, as improvement of the action of many organic compounds used in medicine by activation or biotransformation by metal ions [2–4].

### 3.2. Metal complexes as drugs

In order for the coordination complexes to be approved as drugs, it is necessary to detailed examination of the fundamental aqueous chemistry of the proposed drug, including its pharmacokinetics, the metabolic processes in blood and intracellularly, and the effects of the drug on the target of choice. Inorganic coordination chemistry offers a wide variety of synthesis of coordination compounds with different coordination spheres, including ligand designs, oxidation states and redox potentials of transition metal ions, thus gives the ability to systematically alter the kinetic and thermodynamic properties of the complexes towards biological receptors. Well-known metal ions and their coordination complexes that have found usage in medicine can be divided into:


Platinum complexes are now among the most widely used drugs for the treatment of cancer. Thanks to the successful and widespread use of cisplatin a large number of analogous compounds were synthesized. All these compounds have several common characteristics:


3. the oxidation state of platinum in the complexes is +2 or +4

Today, it is well known that some metal ions are required for normal functions of organism. Lack of zinc, iron, copper, ions and so on can induce disease. Some metal ions such as arsenic, cadmium, chromium, lead and mercury can induce toxicity in humans. Even essential metal ions can be toxic when present in excess. An important aspect of medicinal bioinorganic chemistry is ability to understand all this in the molecular level and treat diseases caused by

Medicinal inorganic chemistry is a multidisciplinary field combining elements of chemistry (synthesis and reactivity), pharmacology (pharmacokinetics and toxicology), biochemistry (targets, structure and conformational changes) and medicinal chemistry (therapeutics, pharmacodynamics and structure-activity relationships). The main focus of this field is to design of novel therapeutic and diagnostic agents and to investigate the mechanism of medicinal action, as improvement of the action of many organic compounds used in medicine by activation or

In order for the coordination complexes to be approved as drugs, it is necessary to detailed examination of the fundamental aqueous chemistry of the proposed drug, including its pharmacokinetics, the metabolic processes in blood and intracellularly, and the effects of the drug on the target of choice. Inorganic coordination chemistry offers a wide variety of synthesis of coordination compounds with different coordination spheres, including ligand designs, oxidation states and redox potentials of transition metal ions, thus gives the ability to systematically alter the kinetic and thermodynamic properties of the complexes towards biological receptors. Well-known metal ions and their coordination complexes that have found usage in medicine

3. Metal-mediated antibiotics like bleomycin, which requires iron or other metals for activity 4. Technetium-99 m and other short-lived isotopes (rhenium-186, rhenium-188 and gallium-68)

Platinum complexes are now among the most widely used drugs for the treatment of cancer. Thanks to the successful and widespread use of cisplatin a large number of analogous com-

2. the general formula of these compounds is cis-[PtA2X2] where A2 are two inert monodentate nitrogen donor ligands or one inert bidentate nitrogen donor ligand, while with X2 are two

pounds were synthesized. All these compounds have several common characteristics:

1. Platinum anticancer agents (e.g., cisplatin, cis-[PtCl2(NH3)2]) 2. The gold(I)-containing antiarthritic drugs (e.g. auranofin)

used as radiopharmaceuticals in disease diagnosis and treatment

6. Antibacterials, antivirals, antiparasitics and radiosensitizing agents

1. bifunctional complex compounds with cis-geometry

labile monodentate or one labile bidentate ligand

5. Magnetic resonance imaging (MRI)-enhancing gadolinium(III) compounds

inadequate metal ion function constitutes.

128 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

biotransformation by metal ions [2–4].

3.2. Metal complexes as drugs

can be divided into:

4. nitrogen-donor ligands have to contain at least one NH bond.

Figure 3 presents some of platinum complexes that are in the medicinal use worldwide.

The second-generation platinum(II) antitumour complexes are carboplatin, oxaliplatin, nonplatinum, zenithplatin, enloplatin, NK121, CI-973 and others. Instead of labile chloride ligands, they contain bidentate ligands such as 1,1-cyclobutanedicarboxylate, glycolate, and complexes with 1,2-diaminocyclohexane as an inert ligand, while the labile ligands are sulphates, malonates and other ligands [5]. The second-generation complexes based on the cisplatin structure were developed to improve toxicity and/or expand the range of useful anticancer activity. The third-generation platinum antitumour complexes are octahedral platinum(IV) coordination compounds with general formula cis-[PtA2X2Y2], where two labile monodentate or one labile bidentate ligand is labeled as Y2. The platinum(IV) drugs are orally administrated to patients. In the presence of various biomolecules such as cysteine or ascorbic acid, the redaction to Pt(II) occurred by leaving the axial ligands Y2. In addition, this group includes new complexes with a trans-geometric structure, polynuclear platinum complexes (BBR3464) and complexes containing a ligand with an asymmetric carbon atom [6].

The mechanism of the antitumour effects of platinum complexes consists in their binding to DNA molecules, thereby preventing replication and transcription of DNA, that is, preventing the process of uncontrolled cell growth [7–9]. From the moment of injection of the drugs in the body to their binding to DNA molecules, a large number of secondary processes happen that

Figure 3. Platinum antitumour complexes with adopted commercial names.

are responsible for the occurrence of toxic effects [8, 9]. Hydrolysis of Pt(II) drugs in the body occurs as a result of a different concentration of chloride ions in and out of the cell. Namely, the high concentration of Cl ion in the extracellular fluid (104 mM) suppresses the hydrolysis process, while in the intracellular low concentration of about 4 mM, it is suitable for the hydrolytic reactions of platinum(II) antitumour drugs [10, 11]. The antitumour platinum(II) agents must not be either too reactive or too inert, since in both cases their toxicity is increasing [10]. On the other hand, the essential characteristic of these compounds must be selectivity towards certain biomolecules [12]. High affinities for the platinum complexes show the biomolecules that contain sulphur, as the thiols and the thioethers, as soft acid platinum(II) drugs form very stable compounds with sulphur donor biomolecules, for example, soft bases. The resulting compounds are responsible for negative side effects during treatment (such as vomiting, resistance, nephrotoxicity, ototoxicity, neurotoxicity and cardiotoxicity).

is linked with angiogenesis, the dynamic process that involves new blood vessel formation. Antitumour activity of ruthenium complexes is related with interaction with proteins in cell

Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

biphenyl, en = ethylenediamine) characterize higher stability to hydrolysis and get in cytoplasm in unchanged form. They are supposed to act as catalysts for glutathione oxidation, which contribute to the increase in cellular oxidative stress and programmed cell death, i.e. apoptosis. Ru(III) complexes tend to be more biologically inert than related Ru(II) complexes, like several other metal ions, can be delivered to cells via the iron transport protein transferrin. The coordination compounds of other metal ions such as Au(I), Au(III), Ti(IV), Cu(II) or MnSOD (manganese-based superoxide dismutase mimics) are on clinical trial. The enzyme superoxide dismutase (SOD), either as the manganese containing MnSOD (present in the mitochondrion) or the dinuclear Cu/Zn-SOD (present in the cytosol and extracellular space),

Polyoxometallates of the Keggin type such as [NaW2lSb29O86][NH4]17 and K12H2[P2W12

Bismuth compounds have been used for their antacid and astringent properties in a variety of gastrointestinal disorders [15, 16]. The effectiveness of bismuth is due to its bacteriocidal action against the Gram-negative bacterium, Helicobacter pylori. Usually, the bismuth preparations are

Injectable Au(I) thiolates and an oral Au(I) phosphine complex are widely used for the treatment of rheumatoid arthritis. Proteins appear to be the targets for gold therapy, including albumin in blood plasma and enzymes in joint tissues. The detection of [Au(CN)2] in the blood and urine of patients undergoing gold therapy (chrysotherapy) raises the possibility that this is an active metabolite. Cyanide could be involved in the metabolic pathways for other metal ions (both natural and therapeutic) in the body since it can be synthesized by some cells. The recent discovery that oxidation of administered Au(I) compounds to Au(III) may be responsible for some of the side effects of gold therapy has highlighted interests in the biological redox chemistry of gold, including possible stabilization of Au(III) complexes by peptides and pro-

Peroxovanadate complexes can inhibit insulin receptor-associated phosphotyrosine phosphatase and activate insulin receptor kinase, and both V(IV) and V(V) compounds offer promise as potential insulin mimics [4, 6]. Lithium compounds are kinetically labile and are used for the treatment of bipolardisorders, and Li(I) inhibition of Mg(II)-dependent inositol monophosphatase enzymes leads to interference with Ca(II) metabolism [4]. Newer uses have appeared in

teins, which now is main target for developing antitumour Au(III) drugs [3, 4, 6].

24H2O have potential in the anti-HIV field, they bind to viral envelope sites on cell surfaces and interfere with virus adsorption [3]. Metal-chelating macrocyclic bicyclam ligands are among the most potent inhibitors of HIV ever described, and there is considerable interest in the role of Zn proteins in the viral life cycle. Metal ions are required for the activity of anti-HIV G-quartet oligonucleotides (antisense oligonucleotides) such as T30177, a potent inhibitor

performs the role of superoxide detoxification in normal cells and tissue [14].

obtained by mixing an inorganic salt with a sugar-like carrier.


http://dx.doi.org/10.5772/intechopen.79472

131

membrane or collagen in extracellular fluid. The ruthenium(II) complexes types [Ru(II)(η<sup>6</sup>

of the enzyme HIV-1 integrase [3].

O48] 

During the recent years, many ruthenium complexes with oxidation state +2 or +3 found to have anticancer activity. Antitumour activities of Ru(II) and Ru(III) complexes take place in a different manner in comparison with platinum(II) drugs, what is linked with geometrical structures and reversible redox potential of ruthenium. The real revolution among the ruthenium complexes was initiated by two isostructural complexes Ru(III): [ImH]trans-[RuCl4(Im)2] and [IndH]trans-[RuCl4(Ind)2], better known by the names ICR and KP1019, respectively (Im = imidazole and Ind = indazole) and [Na]trans-[RuCl4(Im)(dmso-S)] or NAMI-A (dmso-S = dimethyl sulphoxide coordinated through sulphur), which is a structural analogue to the previously synthesized ICR complex (Figure 4). Apart from the fact that these complexes have shown activity to several different types of tumours, it is particularly interesting that they are active against tumours resistant to platinum complexes. The mechanism of action of this compound is not related to DNA binding; rather, it is an antimetastatic agent [13]. Metastasis (the process whereby tumour growth occurs distant from the original or primary tumour site)

Figure 4. The structures of the ruthenium(III) complexes: [ImH]trans-[RuCl4(Im)2] or ICR; [IndH]trans[RuCl4(Ind)2] or KP1019 and ([ImH]trans-[RuCl4(Im)(dmso-S) or NAMI-A.

is linked with angiogenesis, the dynamic process that involves new blood vessel formation. Antitumour activity of ruthenium complexes is related with interaction with proteins in cell membrane or collagen in extracellular fluid.

are responsible for the occurrence of toxic effects [8, 9]. Hydrolysis of Pt(II) drugs in the body occurs as a result of a different concentration of chloride ions in and out of the cell. Namely, the high concentration of Cl ion in the extracellular fluid (104 mM) suppresses the hydrolysis process, while in the intracellular low concentration of about 4 mM, it is suitable for the hydrolytic reactions of platinum(II) antitumour drugs [10, 11]. The antitumour platinum(II) agents must not be either too reactive or too inert, since in both cases their toxicity is increasing [10]. On the other hand, the essential characteristic of these compounds must be selectivity towards certain biomolecules [12]. High affinities for the platinum complexes show the biomolecules that contain sulphur, as the thiols and the thioethers, as soft acid platinum(II) drugs form very stable compounds with sulphur donor biomolecules, for example, soft bases. The resulting compounds are responsible for negative side effects during treatment (such as

130 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

vomiting, resistance, nephrotoxicity, ototoxicity, neurotoxicity and cardiotoxicity).

During the recent years, many ruthenium complexes with oxidation state +2 or +3 found to have anticancer activity. Antitumour activities of Ru(II) and Ru(III) complexes take place in a different manner in comparison with platinum(II) drugs, what is linked with geometrical structures and reversible redox potential of ruthenium. The real revolution among the ruthenium complexes was initiated by two isostructural complexes Ru(III): [ImH]trans-[RuCl4(Im)2] and [IndH]trans-[RuCl4(Ind)2], better known by the names ICR and KP1019, respectively (Im = imidazole and Ind = indazole) and [Na]trans-[RuCl4(Im)(dmso-S)] or NAMI-A (dmso-S = dimethyl sulphoxide coordinated through sulphur), which is a structural analogue to the previously synthesized ICR complex (Figure 4). Apart from the fact that these complexes have shown activity to several different types of tumours, it is particularly interesting that they are active against tumours resistant to platinum complexes. The mechanism of action of this compound is not related to DNA binding; rather, it is an antimetastatic agent [13]. Metastasis (the process whereby tumour growth occurs distant from the original or primary tumour site)

Figure 4. The structures of the ruthenium(III) complexes: [ImH]trans-[RuCl4(Im)2] or ICR; [IndH]trans[RuCl4(Ind)2] or

KP1019 and ([ImH]trans-[RuCl4(Im)(dmso-S) or NAMI-A.

The ruthenium(II) complexes types [Ru(II)(η<sup>6</sup> -arene)(en)X]<sup>+</sup> (X = Cl or I, arene = p-cumene or biphenyl, en = ethylenediamine) characterize higher stability to hydrolysis and get in cytoplasm in unchanged form. They are supposed to act as catalysts for glutathione oxidation, which contribute to the increase in cellular oxidative stress and programmed cell death, i.e. apoptosis. Ru(III) complexes tend to be more biologically inert than related Ru(II) complexes, like several other metal ions, can be delivered to cells via the iron transport protein transferrin.

The coordination compounds of other metal ions such as Au(I), Au(III), Ti(IV), Cu(II) or MnSOD (manganese-based superoxide dismutase mimics) are on clinical trial. The enzyme superoxide dismutase (SOD), either as the manganese containing MnSOD (present in the mitochondrion) or the dinuclear Cu/Zn-SOD (present in the cytosol and extracellular space), performs the role of superoxide detoxification in normal cells and tissue [14].

Polyoxometallates of the Keggin type such as [NaW2lSb29O86][NH4]17 and K12H2[P2W12 O48] 24H2O have potential in the anti-HIV field, they bind to viral envelope sites on cell surfaces and interfere with virus adsorption [3]. Metal-chelating macrocyclic bicyclam ligands are among the most potent inhibitors of HIV ever described, and there is considerable interest in the role of Zn proteins in the viral life cycle. Metal ions are required for the activity of anti-HIV G-quartet oligonucleotides (antisense oligonucleotides) such as T30177, a potent inhibitor of the enzyme HIV-1 integrase [3].

Bismuth compounds have been used for their antacid and astringent properties in a variety of gastrointestinal disorders [15, 16]. The effectiveness of bismuth is due to its bacteriocidal action against the Gram-negative bacterium, Helicobacter pylori. Usually, the bismuth preparations are obtained by mixing an inorganic salt with a sugar-like carrier.

Injectable Au(I) thiolates and an oral Au(I) phosphine complex are widely used for the treatment of rheumatoid arthritis. Proteins appear to be the targets for gold therapy, including albumin in blood plasma and enzymes in joint tissues. The detection of [Au(CN)2] in the blood and urine of patients undergoing gold therapy (chrysotherapy) raises the possibility that this is an active metabolite. Cyanide could be involved in the metabolic pathways for other metal ions (both natural and therapeutic) in the body since it can be synthesized by some cells. The recent discovery that oxidation of administered Au(I) compounds to Au(III) may be responsible for some of the side effects of gold therapy has highlighted interests in the biological redox chemistry of gold, including possible stabilization of Au(III) complexes by peptides and proteins, which now is main target for developing antitumour Au(III) drugs [3, 4, 6].

Peroxovanadate complexes can inhibit insulin receptor-associated phosphotyrosine phosphatase and activate insulin receptor kinase, and both V(IV) and V(V) compounds offer promise as potential insulin mimics [4, 6]. Lithium compounds are kinetically labile and are used for the treatment of bipolardisorders, and Li(I) inhibition of Mg(II)-dependent inositol monophosphatase enzymes leads to interference with Ca(II) metabolism [4]. Newer uses have appeared in the treatment of viral diseases including AIDS, alteration of the immune response and cancer. The lithium salt of linolenic acid (LiGLA) has a significant anticancer effect against certain cancers [6].

4. Investigation of the interactions of zinc(II) and copper(II) complexes

Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

http://dx.doi.org/10.5772/intechopen.79472

Complex compounds are involved in a number of substitution reactions such as ligand exchange, solvent exchange, complexation or anation reactions, solvolysis, acid and base hydrolysis, inter- and intramolecular isomerization, racemization and metal ion exchange [20]. Substitution reactions of complexes can be electrophilic (SE) or nucleophilic (SN) depending on the replacement of either central metal ion or ligand. If the metal ion is substituted during the reaction, that is, electrophile, the reactions are electrophilic substitution (Eq. (4)), otherwise if a

Nucleophilic substitution reactions, according to Langford and Gray, are carried out in three different mechanisms: dissociative (D), associative (A) or interchange mechanism (I) [22]. In the dissociative mechanism (D), the first step of the reaction is dissociation of the one ligand L from the inner coordination sphere, whereby an intermediate with a decreased coordination number forms. In the next step, the entering ligand X binds to the central metal ion. Since the first step of the reaction is slower, it determines the overall rate of the substitution reaction. In the associative mechanism (A), in the first step, the entering ligand X binds to the central metal ion, forming an intermediate with an increased coordination number, and then, in the second step, the leaving ligand L leaves the coordination sphere of the complex. The formation of an intermediate with an increased coordination number is slower and it determines the rates of this substitution process. When an intermediate cannot be detected by kinetic, stereochemical or product distribution studies, the so-called interchange mechanisms (I) are invoked. Associative interchange mechanisms (IA) have rates dependent on the nature of the entering group, whereas dissociative interchange (ID) mechanisms do not. If the process of breaking the bond between the central metal ion and the outgoing ligand L has a greater impact on the rate of reaction, the mechanism is ID, and if the forming a new bond between the central metal ion and the entering ligand X has a greater impact on the chemical reaction rate, the mechanism is

Factors affecting metal ion lability include size, charge, electron configuration and coordination number. The associative mechanism is well known and preferred for four-coordinated square-planar complexes. Dissociative mechanisms are more common for six-coordinated octahedral complexes. Five-coordinated complexes could react in both mechanisms [23]. The study of kinetics and mechanism of the reactions of transition metal complexes expanded with development of organometallic and bioinorganic chemistry, as well as, with the development of new experimental techniques (UV-Vis spectrophotometry, NMR spectroscopy, "stop-flow" spectrophotometry, HPLC, EPR spectroscopy, etc.). The main aims of study are determination of rates of substitution processes, investigation of the influence of different parameters (change

ð4Þ

133

ð5Þ

ligand is replaced that is nucleophilic substitution reaction (Eq. (5)) [21, 22].

with small biomolecules

marked with IA [21, 22].

4.1. Substitution mechanisms in complex compounds

Less labile metal ions can be used to control the levels of biologically active ligands in the body. Thus, Fe(III) in sodium nitroprusside delivers NO to tissues and is used for the treatment of hypertension and control of blood pressure. The possibility arises of utilizing Ru(III) to scavenge NO in the treatment of septic shock. As is mentioned, the injection of gram quantities of Gd(III) complexes to provide contrast magnetic resonance images (MRI) of the body illustrates how the toxicity of metal ions and tissue targeting can be controlled by the appropriate choice of ligands [4]. The importance of metal complexes as imaging agents for various diseases including heart disease and brain disorders have also been recognized. They are able to determine specific aspects of disease such as tissue hypoxia, and can detect molecular phenomenon such as multidrug resistance [3]. Metal centres, being positively charged, favourably bind to negatively charged biomolecules (proteins and nucleic acids) and offer excellent tools for understanding of more specific biological processes including the formation of thrombi and the imaging of infection, and so on. By means of scanning techniques viz. gamma scintigraphy, positron emission tomography (PET) and magnetic resonance imaging (MRI), tissues and organs with radiolabelled compounds can be visualized and such visualization facilitates the detection of abnormalities in their function. Radionuclide complexes are used for diagnosis, as contrast media and as therapeutic agents. A 99mTc radiopharmaceutical (99mTc–SESTAMIBI), known as cardiolite, is an established radiopharmaceutical for myocardial perfusion imaging [3, 4]. A wide variety of coordinated spheres, oxidation states and redox potentials of metal ions in coordinated and organometallic compounds give possibility of design and synthesis of new metal complexes with selected kinetic and thermodynamic properties towards biological receptors [3, 4, 6].

Many Cu-complexes of anti-inflammatory drugs have been found more active in animal models than either their parent Cu(II) salt or NSAID (nonsteroidal anti-inflammatory drugs). Cu(II) complex of salicylate has been found about 30 times more effective than aspirin as an anti-inflammatory agent. The pharmacological activity of these complexes has been proposed to be due to its inherent physico-chemical properties of the complex itself rather than that of its constituents [17].

The amount of metals present in the human body is approximately 0.03% of the body weight. Low metal ion concentrations may be harmful for the body. It has been reported that in various cancers the concentrations of Cd, Cr, Ti, V, Cu, Se and Zn were found to be at a lower level than in normal conditions of body [18]. Ligands having electron donor atoms like N, O, S and P may form coordination bond with metal ion. Chelation causes drastic changes in biological properties of ligands as well as metal moiety and in many cases it causes synergistic effect of metal ion and ligand both [19]. On the other hand, the presence of metals such as lead, mercury, arsenic, uranium and plutonium induces metal poisoning. In order to remove these metals medical procedure, chelation therapy is performing. The medical procedure involves the administration of chelating agents to remove heavy metals from the body. Some common chelating agents are ethylenediaminetetraacetic acid (EDTA), 2,3-dimercaptopropanesulphonic acid (DMPS) and thiamine tetrahydrofurfuryl disulphide (TTFD).

### 4. Investigation of the interactions of zinc(II) and copper(II) complexes with small biomolecules

### 4.1. Substitution mechanisms in complex compounds

the treatment of viral diseases including AIDS, alteration of the immune response and cancer. The lithium salt of linolenic acid (LiGLA) has a significant anticancer effect against certain cancers [6].

132 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

Less labile metal ions can be used to control the levels of biologically active ligands in the body. Thus, Fe(III) in sodium nitroprusside delivers NO to tissues and is used for the treatment of hypertension and control of blood pressure. The possibility arises of utilizing Ru(III) to scavenge NO in the treatment of septic shock. As is mentioned, the injection of gram quantities of Gd(III) complexes to provide contrast magnetic resonance images (MRI) of the body illustrates how the toxicity of metal ions and tissue targeting can be controlled by the appropriate choice of ligands [4]. The importance of metal complexes as imaging agents for various diseases including heart disease and brain disorders have also been recognized. They are able to determine specific aspects of disease such as tissue hypoxia, and can detect molecular phenomenon such as multidrug resistance [3]. Metal centres, being positively charged, favourably bind to negatively charged biomolecules (proteins and nucleic acids) and offer excellent tools for understanding of more specific biological processes including the formation of thrombi and the imaging of infection, and so on. By means of scanning techniques viz. gamma scintigraphy, positron emission tomography (PET) and magnetic resonance imaging (MRI), tissues and organs with radiolabelled compounds can be visualized and such visualization facilitates the detection of abnormalities in their function. Radionuclide complexes are used for diagnosis, as contrast media and as therapeutic agents. A 99mTc radiopharmaceutical (99mTc–SESTAMIBI), known as cardiolite, is an established radiopharmaceutical for myocardial perfusion imaging [3, 4]. A wide variety of coordinated spheres, oxidation states and redox potentials of metal ions in coordinated and organometallic compounds give possibility of design and synthesis of new metal complexes with selected kinetic and thermodynamic properties towards biological

Many Cu-complexes of anti-inflammatory drugs have been found more active in animal models than either their parent Cu(II) salt or NSAID (nonsteroidal anti-inflammatory drugs). Cu(II) complex of salicylate has been found about 30 times more effective than aspirin as an anti-inflammatory agent. The pharmacological activity of these complexes has been proposed to be due to its inherent physico-chemical properties of the complex itself rather than that of its

The amount of metals present in the human body is approximately 0.03% of the body weight. Low metal ion concentrations may be harmful for the body. It has been reported that in various cancers the concentrations of Cd, Cr, Ti, V, Cu, Se and Zn were found to be at a lower level than in normal conditions of body [18]. Ligands having electron donor atoms like N, O, S and P may form coordination bond with metal ion. Chelation causes drastic changes in biological properties of ligands as well as metal moiety and in many cases it causes synergistic effect of metal ion and ligand both [19]. On the other hand, the presence of metals such as lead, mercury, arsenic, uranium and plutonium induces metal poisoning. In order to remove these metals medical procedure, chelation therapy is performing. The medical procedure involves the administration of chelating agents to remove heavy metals from the body. Some common chelating agents are ethylenediaminetetraacetic acid (EDTA), 2,3-dimercaptopropanesulphonic acid (DMPS)

receptors [3, 4, 6].

constituents [17].

and thiamine tetrahydrofurfuryl disulphide (TTFD).

Complex compounds are involved in a number of substitution reactions such as ligand exchange, solvent exchange, complexation or anation reactions, solvolysis, acid and base hydrolysis, inter- and intramolecular isomerization, racemization and metal ion exchange [20]. Substitution reactions of complexes can be electrophilic (SE) or nucleophilic (SN) depending on the replacement of either central metal ion or ligand. If the metal ion is substituted during the reaction, that is, electrophile, the reactions are electrophilic substitution (Eq. (4)), otherwise if a ligand is replaced that is nucleophilic substitution reaction (Eq. (5)) [21, 22].

$$\left[\mathrm{ML}\_{\mathrm{n}}\right] + \left[\mathrm{M} \right] \xrightarrow{\left[\mathrm{ML}\_{\mathrm{n}}\right]} \left[\mathrm{ML}\_{\mathrm{n}}\right] + \mathrm{M} \tag{4}$$

$$\left[\mathbf{M}\mathbf{L}\_{\mathrm{n}}\right] + \mathbf{X} \xleftarrow{\longrightarrow} \left[\mathbf{M}\mathbf{L}\_{\mathrm{n-l}}\mathbf{X}\right] + \mathbf{L} \tag{5}$$

Nucleophilic substitution reactions, according to Langford and Gray, are carried out in three different mechanisms: dissociative (D), associative (A) or interchange mechanism (I) [22]. In the dissociative mechanism (D), the first step of the reaction is dissociation of the one ligand L from the inner coordination sphere, whereby an intermediate with a decreased coordination number forms. In the next step, the entering ligand X binds to the central metal ion. Since the first step of the reaction is slower, it determines the overall rate of the substitution reaction. In the associative mechanism (A), in the first step, the entering ligand X binds to the central metal ion, forming an intermediate with an increased coordination number, and then, in the second step, the leaving ligand L leaves the coordination sphere of the complex. The formation of an intermediate with an increased coordination number is slower and it determines the rates of this substitution process. When an intermediate cannot be detected by kinetic, stereochemical or product distribution studies, the so-called interchange mechanisms (I) are invoked. Associative interchange mechanisms (IA) have rates dependent on the nature of the entering group, whereas dissociative interchange (ID) mechanisms do not. If the process of breaking the bond between the central metal ion and the outgoing ligand L has a greater impact on the rate of reaction, the mechanism is ID, and if the forming a new bond between the central metal ion and the entering ligand X has a greater impact on the chemical reaction rate, the mechanism is marked with IA [21, 22].

Factors affecting metal ion lability include size, charge, electron configuration and coordination number. The associative mechanism is well known and preferred for four-coordinated square-planar complexes. Dissociative mechanisms are more common for six-coordinated octahedral complexes. Five-coordinated complexes could react in both mechanisms [23]. The study of kinetics and mechanism of the reactions of transition metal complexes expanded with development of organometallic and bioinorganic chemistry, as well as, with the development of new experimental techniques (UV-Vis spectrophotometry, NMR spectroscopy, "stop-flow" spectrophotometry, HPLC, EPR spectroscopy, etc.). The main aims of study are determination of rates of substitution processes, investigation of the influence of different parameters (change of reactant concentration, pH, temperature and pressure change, introduction catalyst, etc.), investigation of interactions between potential antitumour metal-based drugs and biologically relevant molecules [20–24].

### 4.2. The bioinorganic chemistry of zinc(II) and copper(II) complexes

Transition metal compounds play crucial roles as cofactors in metalloproteins; they act mainly as a Lewis acid. Two essential metal ions, namely zinc and copper ions, modulate enzymes activities, catalytic and regulatory functions, oxidative-reductive processes, etc. [4, 24]. Zinc has a specific role in bioinorganic processes because of the peculiar properties of the coordination compounds of the zinc(II) ion, easily can be four-, five- or six-coordinate, without a marked preference for six coordination [24]. The most studied metalloproteins in which zinc serves a structural role belong to the zinc-finger family, which is involved in control of nucleic acid replication, transcription and repair [25]. In zinc-finger proteins, zinc is tetrahedrally coordinate to histidines and/or cysteines, the coordination of aspartic acid and glutamic acid residues to the metal, also has been found in metalloenzymes [26].

> The mechanism of potential anticancer activity of zinc(II) complexes is assumed to be connected with: (i) fast inter conversion among its four-, five- and six-coordinate states; (ii) preference of the variable coordination geometries (tetrahedral, five-coordinate and octahedral) that zinc(II) is able to adopt, towards diverse donor site of relevant nucleophiles [27]. Knowledge of mechanism of the interaction zinc(II) ions with biomolecules and other relevant ligands is essential for understanding the processes in the cells during delivery of complexes to

> Figure 5. Titrations of [ZnCl2(terpy)] with imidiazole as monitored by UV-vis spectra. Left: [ZnCl2(terpy)]-imidiazole,

Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

http://dx.doi.org/10.5772/intechopen.79472

135

On the other hand, copper(II) controls cancer development. It serves as a limiting factor for multiple aspects of tumour progression, growth, angiogenesis and metastasis [34–36]. Copper(II) complexes offer various potential advantages as antimicrobial, antiviral, anti-inflammatory, antitumour agents, enzyme inhibitor, chemical nucleases, and they are also beneficial against several diseases like copper rheumatoid and gastric ulcers [37, 38]. It has been shown recently that metal complexes of imidazole terpyridine (itpy) have potential applications in chemotherapy [39]. Changing the ligand environment towards the specific target is a possible way of tuning the selectivity of a drug molecule. The nature of the ligands plays an important role in the binding of a

The chemistry of copper is dominated by the +2 oxidation state, for example, copper(II) complex ions. In comparison with other divalent first-row transition-metal aqua ions, the

tion. As a result of the d<sup>9</sup> electronic configuration, an elongation of the axial-bound solvent molecules is weakly coordinated. Due to this distortion the axial water molecules are weaker bound to the central atom and therefore can be more easily substituted. The strong ligand field

ligands (tren) will restrict the degree and rapidity of distortion of the [Cu(tren)H2O]2+ complex and remove the dynamic Jahn-Teller effect as stabilizing effect [43]. The bulk of five-coordinate

2+ ion is extremely labile [41, 42]. This effect is a consequence of Jahn-Teller distor-


:60

,200-triaminotriethylamine

,200- terpyridine or derivative,

metal complex to a biomolecule such as DNA or protein [39, 40].

right: cross-section of UV-vis spectra at 350 nm in the presence of 0.001 M NaCl [31].

forces the metal ion into a different geometry, for example, 2,2<sup>0</sup>

{Cu(terpy)(bipy)} and {Cu(terpy)(phen)} (terpy = 2,2<sup>0</sup>

DNA and proteins.

[Cu(H2O)6]

bipy = 2,2<sup>0</sup>

Zinc is a good intermediate Lewis acid, especially in complexes with lower coordination numbers; it lowers the pKa of coordinated water and is kinetically labile, and the inter conversion among its four-, five- and six-coordinate states is fast [27]. The theoretical studies have shown that zinc does not have a strong preference for a particular number of water molecules in its first coordination layer and can accommodate four, five or six water ligands; the calculated energy differences between isomeric [Zn(H2O)6] 2+, {[Zn(H2O)5]'(H2O)}2+ and {[Zn(H2O)4]'(H2O)2} 2+ complexes differ by only a few kilocalories per mole [28]. Moreover, dynamic conversion of structural zinc into a transient catalytic centre may be a mechanism for nucleic acid cleavage [29]. Recently, we determined the metal–ligand stoichiometry between [ZnCl2(en)] (where en = 1,2-diaminoethane or ethylenediamine) complex and chloride at pH 7.2. In the presence of an excess of chloride (0.010 M NaCl), the octahedral [ZnCl4(en)]2� formed in solution at pH 7.2 [30] (Eq. (6)).

Also, we have investigated the mechanism of interaction between biologically relevant nucleophiles and [ZnCl2(terpy)] (where terpy = 2,2<sup>0</sup> :60 ,200- terpyridine) complex in the presence of NaCl [31, 32]. The excess of chloride did not affect coordination geometry of complex [32]. The result of the metal–ligand stoichiometry between [ZnCl2(terpy)] complex and imidazole implied formation of the five-coordinate specie [Zn(terpy)(imidazole)2] [31] (Figure 5).

Promising anticancer agents could be the zinc-based compounds, especially because zinc is implicated as an important cytotoxic/tumour suppressor agent in several cancers [33].

of reactant concentration, pH, temperature and pressure change, introduction catalyst, etc.), investigation of interactions between potential antitumour metal-based drugs and biologically

Transition metal compounds play crucial roles as cofactors in metalloproteins; they act mainly as a Lewis acid. Two essential metal ions, namely zinc and copper ions, modulate enzymes activities, catalytic and regulatory functions, oxidative-reductive processes, etc. [4, 24]. Zinc has a specific role in bioinorganic processes because of the peculiar properties of the coordination compounds of the zinc(II) ion, easily can be four-, five- or six-coordinate, without a marked preference for six coordination [24]. The most studied metalloproteins in which zinc serves a structural role belong to the zinc-finger family, which is involved in control of nucleic acid replication, transcription and repair [25]. In zinc-finger proteins, zinc is tetrahedrally coordinate to histidines and/or cysteines, the coordination of aspartic acid and glutamic acid

Zinc is a good intermediate Lewis acid, especially in complexes with lower coordination numbers; it lowers the pKa of coordinated water and is kinetically labile, and the inter conversion among its four-, five- and six-coordinate states is fast [27]. The theoretical studies have shown that zinc does not have a strong preference for a particular number of water molecules in its first coordination layer and can accommodate four, five or six water ligands; the calculated energy differences

by only a few kilocalories per mole [28]. Moreover, dynamic conversion of structural zinc into a transient catalytic centre may be a mechanism for nucleic acid cleavage [29]. Recently, we determined the metal–ligand stoichiometry between [ZnCl2(en)] (where en = 1,2-diaminoethane or ethylenediamine) complex and chloride at pH 7.2. In the presence of an excess of chloride

Also, we have investigated the mechanism of interaction between biologically relevant

ence of NaCl [31, 32]. The excess of chloride did not affect coordination geometry of complex [32]. The result of the metal–ligand stoichiometry between [ZnCl2(terpy)] complex and imidazole implied formation of the five-coordinate specie [Zn(terpy)(imidazole)2] [31]

Promising anticancer agents could be the zinc-based compounds, especially because zinc is implicated as an important cytotoxic/tumour suppressor agent in several cancers [33].

:60

(0.010 M NaCl), the octahedral [ZnCl4(en)]2� formed in solution at pH 7.2 [30] (Eq. (6)).

2+, {[Zn(H2O)5]'(H2O)}2+ and {[Zn(H2O)4]'(H2O)2}

2+ complexes differ

,200- terpyridine) complex in the pres-

ð6Þ

4.2. The bioinorganic chemistry of zinc(II) and copper(II) complexes

134 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

residues to the metal, also has been found in metalloenzymes [26].

nucleophiles and [ZnCl2(terpy)] (where terpy = 2,2<sup>0</sup>

relevant molecules [20–24].

between isomeric [Zn(H2O)6]

(Figure 5).

Figure 5. Titrations of [ZnCl2(terpy)] with imidiazole as monitored by UV-vis spectra. Left: [ZnCl2(terpy)]-imidiazole, right: cross-section of UV-vis spectra at 350 nm in the presence of 0.001 M NaCl [31].

The mechanism of potential anticancer activity of zinc(II) complexes is assumed to be connected with: (i) fast inter conversion among its four-, five- and six-coordinate states; (ii) preference of the variable coordination geometries (tetrahedral, five-coordinate and octahedral) that zinc(II) is able to adopt, towards diverse donor site of relevant nucleophiles [27]. Knowledge of mechanism of the interaction zinc(II) ions with biomolecules and other relevant ligands is essential for understanding the processes in the cells during delivery of complexes to DNA and proteins.

On the other hand, copper(II) controls cancer development. It serves as a limiting factor for multiple aspects of tumour progression, growth, angiogenesis and metastasis [34–36]. Copper(II) complexes offer various potential advantages as antimicrobial, antiviral, anti-inflammatory, antitumour agents, enzyme inhibitor, chemical nucleases, and they are also beneficial against several diseases like copper rheumatoid and gastric ulcers [37, 38]. It has been shown recently that metal complexes of imidazole terpyridine (itpy) have potential applications in chemotherapy [39]. Changing the ligand environment towards the specific target is a possible way of tuning the selectivity of a drug molecule. The nature of the ligands plays an important role in the binding of a metal complex to a biomolecule such as DNA or protein [39, 40].

The chemistry of copper is dominated by the +2 oxidation state, for example, copper(II) complex ions. In comparison with other divalent first-row transition-metal aqua ions, the [Cu(H2O)6] 2+ ion is extremely labile [41, 42]. This effect is a consequence of Jahn-Teller distortion. As a result of the d<sup>9</sup> electronic configuration, an elongation of the axial-bound solvent molecules is weakly coordinated. Due to this distortion the axial water molecules are weaker bound to the central atom and therefore can be more easily substituted. The strong ligand field forces the metal ion into a different geometry, for example, 2,2<sup>0</sup> ,200-triaminotriethylamine ligands (tren) will restrict the degree and rapidity of distortion of the [Cu(tren)H2O]2+ complex and remove the dynamic Jahn-Teller effect as stabilizing effect [43]. The bulk of five-coordinate {Cu(terpy)(bipy)} and {Cu(terpy)(phen)} (terpy = 2,2<sup>0</sup> :60 ,200- terpyridine or derivative, bipy = 2,2<sup>0</sup> -bipyridine or derivative, phen = 1,10-phenanthroline or derivative) complexes

exhibiting ostensibly square-based pyramidal geometries also shows an additional interaction in the remaining axial site leading to a better description as their being six-coordinate [44].

complex and biological relevant nucleophiles such as 5<sup>0</sup>

DL-Asp > 5<sup>0</sup>

GMP [30].

GSH > 5<sup>0</sup>

pH 7.4, at 300 K [45].


ity is: DL-Asp > L-Met > GSH > 5<sup>0</sup>

of reactivity of the investigated nucleophiles for the first reaction step is 5<sup>0</sup>


pyramidal geometry in the presence of the buffer or bio-ligands [45].



Mechanism of Interactions of Zinc(II) and Copper(II) Complexes with Small Biomolecules

Asp was followed under pseudo-first-order conditions by UV-Vis spectrophotometry. In the presence of an excess of chloride the octahedral complex anion [ZnCl4(en)]<sup>2</sup>� has been formed. The first step of the substitution reactions could be interpreted as substitution of the axial chlorido ligands in cis position towards bidentate ethylenediamine by the biologically relevant nucleophiles, while the second step is substitution of the equatorial chlorido ligand. The order

In the presence of an excess of chloride, the square-planar complex [CuCl2(en)] exists as a pseudo octahedral complex with two axially and weakly-bound solvent ligands, these ligands are rapidly replaced/substituted by chloride ions to form [CuCl4(en)]<sup>2</sup>� as a pre-equilibrium intermediate, while equilibrium reaction was observed for [CuCl2(terpy)] [45]. The order of reactivity of the investigated bio-relevant nucleophiles towards [CuCl4(en)]<sup>2</sup>� complex is:

reactivity of biomolecules towards [CuCl2(en)] and [CuCl2(terpy)] complexes could be explained by different geometrical structures of complexes (octahedral and square-pyramidal, respectively) in the presence of chloride and their different preferences towards donor atoms of biomolecules. Mass spectrum of [CuCl2(terpy)] in Hepes buffer has shown two new signals at m/z = 477.150 and m/z = 521.00, assigned to [CuCl(terpy)]<sup>+</sup> � Hepes fragments of coordinated Hepes buffer. These signals also appear in mass spectra of ligand-substitution reactions between [CuCl2(terpy)] and biomolecules in molar ratio 1:1 and 1:2. According to EPR data, L-Met forms the most stable complex with [CuCl2(en)] among the ligands considered (Figure 6), while [CuCl2(terpy)] complex did not show significant changes in its square-

Figure 6. Left: EPR spectrum of 0.0001 M [CuCl2(en)] complex solution in 0.010 M NaCl 0.025 M Hepes buffer, pH 7.4, at 300 K. Right: EPR spectrum of 0.0001 M equimolar [CuCl2(en)] � L-Met solution in 0.0010 M NaCl 0.025 M Hepes buffer,




http://dx.doi.org/10.5772/intechopen.79472




137

### 4.3. Study on the kinetics and mechanism of the reactions of zinc(II) and copper(II) complex compounds with relevant biomolecules

Clear understanding of complex formation reactions between zinc(II) and copper(II) complexes and biorelevant nucleophiles is still largely missing. Substitution behaviour of Zn and Cu-complex compounds at physiological conditions is very complex due to the rather high molecular mobility, distortions of complex compound and facile interconversion of four- to five-, six-coordinate complexes. Adopted geometry of complex compounds conditionals different preferences towards bio-ligands. Thus, the square pyramidal structure of Zn(II) and Cu(II) in biological systems prefers O-carboxylate or carbonyl and N-imidazole donor bio-ligand, while tetrahedral prefers S-thiolate or thioether, N-imidazole [4]. Investigation of mechanism of the interaction between zinc(II) and copper(II) ions and biomolecules and other relevant ligands is essential for understanding the mechanism of action of the potential zinc- and copper-based antitumour drugs.

Recently, we have investigated by different methods (UV-Vis, EPR, HPLC-MS, densityfunctional calculations, mole-ratio, etc.) the kinetics and mechanism of the reactions between tetrahedral and square-pyramidal Zn(II) or Cu(II) complex compounds (i.e. [ZnCl2(en)], [CuCl2(en)], [CuCl2(terpy)] and [ZnCl2(terpy)]) with bio-nucleophiles as a function of entering nucleophile concentrations and temperature at pH 7.2–7.4 [30–32, 45]. The kinetics showed that the substitution reactions involve the consecutive displacement of both chloride ligands for every complex. Higher reactivity of [CuCl2(terpy)] complex then [ZnCl2(terpy)] was obtained. The order of reactivity of the investigated biomolecules for the first reaction step is: glutathione (GSH) >> DL-aspartic acid (DL-Asp) > guanosine-5<sup>0</sup> -monophosphate (5<sup>0</sup> -GMP) > inosine-5<sup>0</sup> -monophosphate (5<sup>0</sup> -IMP) > L-methionine (L-Met) (for [CuCl2(terpy)]), while for [ZnCl2(terpy)] order is: DL-Asp > GSH > 5<sup>0</sup> -GMP > 5<sup>0</sup> -IMP >> L-Met. Chelate formation and pre-equilibrium were obtained for the substitution process between [ZnCl2(terpy)] complex and glutathione [32]. The π-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern [31, 32, 45]. Based on energetic stability of complexes, it can be concluded that both complexes make hydrates very easy, but the bond between water molecule and metal ion is pretty weak. In addition, there is a very good agreement between experimental and calculated spectra obtained for hydrated and non-hydrated complexes in aqueous solution. During formation of monohydrate, Zn(II) and Cu(II) complexes obtain little shaped octahedral geometry, with three nitrogen and chloride atom in the central plane, and with water molecule and the other chloride atom on the line almost normal to the plane [32]. The presence of various concentration of chlorides has significant impact on rate constants of substitution processes of the [ZnCl2(terpy)] complex by nucleophiles [31].

As we mentioned in Section 4.2, the mole-ratio method was used for determining the metalligand stoichiometry between [ZnCl2(en)] and chloride at pH 7.2. The results have shown stepwise formation of 1:1 and 1:2 complexes, and indicate additional coordination of chlorides in the first coordination sphere (Eq. (6)) [30]. The kinetics of ligand substitution reactions of this complex and biological relevant nucleophiles such as 5<sup>0</sup> -IMP, 5<sup>0</sup> -GMP, L-Met, GSH and DL-Asp was followed under pseudo-first-order conditions by UV-Vis spectrophotometry. In the presence of an excess of chloride the octahedral complex anion [ZnCl4(en)]<sup>2</sup>� has been formed. The first step of the substitution reactions could be interpreted as substitution of the axial chlorido ligands in cis position towards bidentate ethylenediamine by the biologically relevant nucleophiles, while the second step is substitution of the equatorial chlorido ligand. The order of reactivity of the investigated nucleophiles for the first reaction step is 5<sup>0</sup> -IMP > GSH > L-Met > DL-Asp > 5<sup>0</sup> -GMP, while for the second reaction step is GSH > L-Met > 5<sup>0</sup> -IMP > DL-Asp > 5<sup>0</sup> - GMP [30].

exhibiting ostensibly square-based pyramidal geometries also shows an additional interaction in the remaining axial site leading to a better description as their being six-coordinate [44].

4.3. Study on the kinetics and mechanism of the reactions of zinc(II) and copper(II) complex

Clear understanding of complex formation reactions between zinc(II) and copper(II) complexes and biorelevant nucleophiles is still largely missing. Substitution behaviour of Zn and Cu-complex compounds at physiological conditions is very complex due to the rather high molecular mobility, distortions of complex compound and facile interconversion of four- to five-, six-coordinate complexes. Adopted geometry of complex compounds conditionals different preferences towards bio-ligands. Thus, the square pyramidal structure of Zn(II) and Cu(II) in biological systems prefers O-carboxylate or carbonyl and N-imidazole donor bio-ligand, while tetrahedral prefers S-thiolate or thioether, N-imidazole [4]. Investigation of mechanism of the interaction between zinc(II) and copper(II) ions and biomolecules and other relevant ligands is essential for understanding the mechanism of action of the potential zinc- and copper-based

Recently, we have investigated by different methods (UV-Vis, EPR, HPLC-MS, densityfunctional calculations, mole-ratio, etc.) the kinetics and mechanism of the reactions between tetrahedral and square-pyramidal Zn(II) or Cu(II) complex compounds (i.e. [ZnCl2(en)], [CuCl2(en)], [CuCl2(terpy)] and [ZnCl2(terpy)]) with bio-nucleophiles as a function of entering nucleophile concentrations and temperature at pH 7.2–7.4 [30–32, 45]. The kinetics showed that the substitution reactions involve the consecutive displacement of both chloride ligands for every complex. Higher reactivity of [CuCl2(terpy)] complex then [ZnCl2(terpy)] was obtained. The order of reactivity of the investigated biomolecules for the first reaction step is:


pre-equilibrium were obtained for the substitution process between [ZnCl2(terpy)] complex and glutathione [32]. The π-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern [31, 32, 45]. Based on energetic stability of complexes, it can be concluded that both complexes make hydrates very easy, but the bond between water molecule and metal ion is pretty weak. In addition, there is a very good agreement between experimental and calculated spectra obtained for hydrated and non-hydrated complexes in aqueous solution. During formation of monohydrate, Zn(II) and Cu(II) complexes obtain little shaped octahedral geometry, with three nitrogen and chloride atom in the central plane, and with water molecule and the other chloride atom on the line almost normal to the plane [32]. The presence of various concentration of chlorides has significant impact on rate constants of

As we mentioned in Section 4.2, the mole-ratio method was used for determining the metalligand stoichiometry between [ZnCl2(en)] and chloride at pH 7.2. The results have shown stepwise formation of 1:1 and 1:2 complexes, and indicate additional coordination of chlorides in the first coordination sphere (Eq. (6)) [30]. The kinetics of ligand substitution reactions of this





glutathione (GSH) >> DL-aspartic acid (DL-Asp) > guanosine-5<sup>0</sup>

substitution processes of the [ZnCl2(terpy)] complex by nucleophiles [31].

compounds with relevant biomolecules

136 Basic Concepts Viewed from Frontier in Inorganic Coordination Chemistry

antitumour drugs.

sine-5<sup>0</sup>


[ZnCl2(terpy)] order is: DL-Asp > GSH > 5<sup>0</sup>

In the presence of an excess of chloride, the square-planar complex [CuCl2(en)] exists as a pseudo octahedral complex with two axially and weakly-bound solvent ligands, these ligands are rapidly replaced/substituted by chloride ions to form [CuCl4(en)]<sup>2</sup>� as a pre-equilibrium intermediate, while equilibrium reaction was observed for [CuCl2(terpy)] [45]. The order of reactivity of the investigated bio-relevant nucleophiles towards [CuCl4(en)]<sup>2</sup>� complex is: GSH > 5<sup>0</sup> -GMP > 5<sup>0</sup> -IMP > DL-Asp > L-Met, while towards [CuCl2(terpy)] the order of reactivity is: DL-Asp > L-Met > GSH > 5<sup>0</sup> -GMP > 5<sup>0</sup> -IMP, for the first reaction step. Different order of reactivity of biomolecules towards [CuCl2(en)] and [CuCl2(terpy)] complexes could be explained by different geometrical structures of complexes (octahedral and square-pyramidal, respectively) in the presence of chloride and their different preferences towards donor atoms of biomolecules. Mass spectrum of [CuCl2(terpy)] in Hepes buffer has shown two new signals at m/z = 477.150 and m/z = 521.00, assigned to [CuCl(terpy)]<sup>+</sup> � Hepes fragments of coordinated Hepes buffer. These signals also appear in mass spectra of ligand-substitution reactions between [CuCl2(terpy)] and biomolecules in molar ratio 1:1 and 1:2. According to EPR data, L-Met forms the most stable complex with [CuCl2(en)] among the ligands considered (Figure 6), while [CuCl2(terpy)] complex did not show significant changes in its squarepyramidal geometry in the presence of the buffer or bio-ligands [45].

Figure 6. Left: EPR spectrum of 0.0001 M [CuCl2(en)] complex solution in 0.010 M NaCl 0.025 M Hepes buffer, pH 7.4, at 300 K. Right: EPR spectrum of 0.0001 M equimolar [CuCl2(en)] � L-Met solution in 0.0010 M NaCl 0.025 M Hepes buffer, pH 7.4, at 300 K [45].
