**5. Coordination chemistry of heavier group 13 metal(I) halides**

Aluminium and gallium are the two categories of metal monohalides in Group-13 (**Figure 2**). The organometallic chemistry of these two monohalides of Al and Ga-based molecules is an expeditiously bolster object of research. The complexities in synthesising these compounds and the new situations in which they will be used as synthetic starting materials attracted a great deal of interest. From a theoretical perspective, these molecules that correspond to electronic and molecular structure and bonding have a lot of significance [5]. It has been reported that by co-condensing AlX with combinations of donor solvents and toluene, metastable solutions of aluminium monohalides can be made [6, 7, 59, 79, 80]. The AlX adducts [(EEt3)AlX]4 (X = Br, I E = N, P) were confined and geometrically characterised [74, 96–99]. The symmetrical bridging dimers, which were observable in lowtemperature environments [25, 26], contradicted with the spectroscopic investigation of the vapour phase AlX (X = F − I) [74, 76]. The reactivity of monovalent Al compounds is high [100–103], and their scope exceeding that of the more reactive transition metal complexes. Fischer et al. described the chemical [(CO)4Fe- Al(η<sup>5</sup> - C5Me5)], which was synthesised from an AlCp\* unit that was bonded to a metal via a terminal non-bridging bond. When compared to dialkyl fragment ER2 [43–45], single-source precursors of M-M bonds with ER fragments were significant in attaining the molecular force of the thin film stoichiometry [104–110]. The metal complexes (CO)nME[(X)L2] of Cr, Mo, W, and Fe with the monohalides of aluminium and gallium ligands were explored in depth. The nature of the bonding in the donor-stabilised complexes [(NH3)2(CO)5W(ECl)] for group-13 atoms (E = B-Tl) was examined [110].

The structural and bonding study of monohalide of group 13 elements as a ligand in metal carbonyls was recently examined (**Figure 3**) [18–21]. Lewis bases such as [(tmeda)(CO)4Fe(GaCl)] assist in the stabilisation of GaX ligands in complexes, however, ligands such as GaR are not supported by Lewis bases [111–114]. The great reactivity of GaX complexes, as well as the scarcity of EX synthons, present difficulties in their synthesis. Aldridge et al. (2008) addressed these obstacles by employing a sterically hindered, rich-electron metalcore as well as a stable GaI2 precursor [85, 86]. M-E complexes are more interesting because of their binding properties [115]. Bond dissociation energies for such complexes have been determined thermodynamically, in the same way, that they have been determined for boron monohalide ligands [116]. As a result, BDEs for [(CO)4Fe(GaX)] has been calculated [X = F-I(equatorial/axial): 140.6/141.8;151.5/151.0;153.6/152.7; 158.6/157.7 kJmol−1 respectively] to be 193.7 kJ mol−1 (for CO equatorial/axial) lower than for [Fe(CO)5] but 91.6/88.7 kJ mol−1 (for N2 equatorial/axial) less than [Fe(CO)4(N2)]. Because various terminal metal gallylene, alumylene, and borylene compounds have been computationally investigated and reported [84, 117–120], very few complexes such as dialkyl, haloaryl, and dihalogallylene complexes have been synthesised and characterised. Moreover, while the covalent to electrostatic

**Figure 2.** *Development of aluminium monohalides based complexes.*

**Figure 3.**

*Development of gallium monohalides based complexes.*

#### **Figure 4.**

*Development of indium monohalides based complexes.*

interaction ratios obtained for Fe–GaX bonds are analogous to those calculated for Fe–CO and Fe–N2, the impact of sigma donation to the covalent bonding contribution is substantially larger for GaX than for Fe–CO and Fe–N2 bonds [115]. Although a variety of systems including these lengths EX bridging between two metal centres have previously been characterised, complexes containing the heavier EX as terminal ligands have just previously been reported experimentally [85–87]. Dimeric complexes of the kind [(LnM)2E(μ-X)2E(MLn)2] or polymeric structure with sterically less hindered transition metal fragments [84, 121–125] are examples. The Lewis base coordinated at the group 13 centre can be used to segregate mononuclear systems in such circumstances [121, 126–128]. Pandey et al. (2010) investigated the bonding nature of group 8 and 10 metal complexes with dihalogallyl ligands. As a result, understanding the bonding behaviour of M-Ga in gallyl complexes is significant. The binding behaviour of the M-E bond in charge-neutral compounds coupled with typical metal carbonyls such as chromium hexacarbonyl, iron pentacarbonyl and nickel tetracarbonyl was systematically investigated to gain a greater understanding [69, 72, 73, 129–131].

Many researchers are still interested in synthesising metal coordination with In ligands, which has been recognised in recent decades [132–134] (**Figure 4**). The cationic derivative [Cp\*Fe (GaCl)(phen) (CO)2] + was produced as the [BPh4] − salt by reacting [Cp\*Fe(CO)2(GaCl2)] with Na[BPh4] in the presence of 1,10-phenanthroline [135]. The chemistry of halide abstraction technic was initially utilised to synthesise the trimetallic indium and gallium cations in 2004 and was later extended to synthesise the iodogallylene complex [Cp\*Fe(GaI)(dppe)]+ [ArBF4] − by using a more electronegative and sterically hindered bis(phosphine)iron fragment [85–87, 136, 137]. Gallium has a low coordination number, which is compatible with crystallographic parameters. For the similar bivalent ligand system in [(OC)4Fe(Ga-C6H5)(GaI)], the bond distance of Fe–Ga, Ga–I and Fe–Ga were 2.222, 2.444, 2.225 Å respectively and the bond angle of Fe–Ga–I was 171.40 [85, 115]. The weaker orbital contribution is thought to reflect the more diffuse nature of the 4s/4p orbitals derived from gallium, as well as less effective interactions with the fragment orbitals of [CpFe(dmpe)]+ , despite the higher energy of the HOMO for GaI (−6.08 eV vs. -9.03 eV for CO) and greater localization of the LUMO at the donor atom. The total metal–ligand bond strengths [Δ*E*int = −103 (GaI), −285 (BF), −213 (CO), and − 120 kJmol−1 (N2)] after adding CO to form [Cp\*Fe(dppe)(CO)]+ [ArBF4] − reveal very poor binding of the

### *Application of Density Functional Theory in Coordination Chemistry: A Case Study of Group 13… DOI: http://dx.doi.org/10.5772/intechopen.99790*

GaI ligand [52]. The Rh–Ga distance [2.471 Å] becomes noticeably shorter [2.334 Å] when pyridine is coordinated at the gallium atom, this is in marked contradiction to analogous borylene systems, and it appears to indicate that the gallylene ligand is a stronger σ -donor, as well as the system's relative lack of π-back bonding representation. The availability of low valent and highly reactive [InI R] molecules allowed for the synthesis of many complexes. [InI R] molecules often operate as two-electron donors for metal fragments in both terminal and bridging modes of coordination [138–140]. The reason that identical complexes are coupled via numerous bonds (LnM-EX) reflects not only the significant structural and bonding challenges brought by such complexes but also the insufficiency of strong experimental verification of possible bonding models [115, 141]. Mays *et al.* established the synthesis of the [(ɳ<sup>5</sup> -C5H5)2Fe(CO)2(InCl)] complex via the addition of InCl into the Fe-Fe bond of [Fe2(CO)4(ɳC5H5)2] [142]. In most common organic solvents, commercially available monovalent indium compounds disintegrate or become insoluble [4, 71]. In this context, many studies have proposed a protonolytic approach to monovalent indium sources to improve performance and stability. The synthesis of stable metal complexes, particularly with group-13 diyl ligands ER, involved a detailed study of the bonds between the compounds. It's unusual to find a perfect computational analysis that accurately describes the bonding of the M-ERn [68]. The ability of tri-coordinate complexes of the type [LnM]2(μ-EX) to oligomerize through E-X-E bridges are modified by larger additional ligands of electrostatic repulsion induced by the net charge of [(OC)5Cr2(μ-EX)]2 where E = In; X = F-I [84, 136, 137, 143]. For the Cp\*Fe(CO)2 systems, synthetic pathways initiating in EI or EIII precursors are feasible (using addition or salt elimination techniques), with monomeric complexes generated that differ from the oligo/polymeric structures of similar [Cp\*Fe(CO)2] complexes [84, 123, 144, 145]. Although the reaction of [Cp\*Fe(CO)2]2 and InI have clear mechanistic similarities to classical oxidative insertion reactions, the assignment of oxidation states in the product is rather arbitrary because iron and indium have similar electronegativities (1.83 and 1.78 on the Pauling scale, respectively).
