**3.6 Steric effect**

comparison studies revealed that the metal complex formed from chelating ligands are thermodynamically more stable than the complex formed from monodentate ligand. For example, formation of complexes from hydrated cadmium ion,

<sup>2</sup><sup>þ</sup> <sup>&</sup>lt; ½ � Cd en ð Þ <sup>2</sup><sup>þ</sup> <sup>&</sup>lt;½ � Cd trien ð Þ <sup>2</sup><sup>þ</sup>

A macrocyclic ligand is a cyclic molecule that contains nine or more atoms in

Resonance increases the stability of the complexes. For example, acetylacetonate

anion ligand shows resonance, and as a result it forms stable complexes upon reacting with metal ion (**Figure 1**). The ligand-metal π bonding increases the delocalization of electrons compared to free enolate as shown below and leads to

the cyclic structure and has three or more potential donor atoms which can coordinate to the metal ion. It has been observed that the stability of metal complexes in the presence of macrocyclic ligand of appropriate size is higher than the stability of complexes coordinated to open-ended multidentate chelating ligands. Some notable examples of macrocyclic ligands include cyclic crown

netetramine (trien), and their stability is in the following order:

Cd CH ð Þ 3NH2 <sup>4</sup>

*Stability and Applications of Coordination Compounds*

2+ with methylamine (CH3NH2), ethylenediamine (en) and triethyle-

[Cd(H2O)4]

**3.4 Macrocyclic effect**

polyether, heme, etc. [12].

increased stability (**Figure 2**).

*Resonance structure of acetonylacetonate ligand.*

**3.5 Resonance effect**

**Figure 1.**

**Figure 2.**

**30**

*Acetonylacetonate-metal complex.*

The presence of bulky substituents in the ligands can affect the stability of the metal complex, and this type of destabilization of metal complex due to bulkiness of the substituent is called as steric effect [13]. For example, consider the ligand 8-hydroxy quinoline and its methyl substituted derivative 2-methyl-8-hydroxy quinolone. Both are bidentate ligands and form chelated complexes with Ni2+ ion as shown in **Figure 3**.

**Figure 3.** *Chelating complexes of Ni(II) ion showing steric effect.*

The complex (II) is less stable than complex (I) because of bulky group attached to an atom adjacent to donor atom which cause a steric hindrance and lower the stability of the complex.

#### **3.7 Crystal field stabilization energy (CFSE)**

The crystal field stabilization energy (CFSE) is one of the most important factors that decides the stability of the metal complexes. CFSE is the stability that arises when a metal ion coordinates to a set of ligands, which is due to the generation of a crystal field by the ligands. Thus, a higher value of CFSE means that the complex is thermodynamically stable and kinetically inert. Some of the notable examples of complexes that have high CFSE are low spin 5d6 complexes of Pt4+ and Ir3+ and square planar 5d8 complexes of Pt2+. All these complexes are thermodynamically stable and kinetically inert, which undergo ligand substitution reactions extremely slowly [3, 4].
