What is meant by chelating effect?

A bidentate (or) a poly dentate ligand is known as chelating ligand if on coordination it results in the formation of a closed (or) cyclic structure.Chelating ligands form more stable complexes than the monodentate ligands. This is called chelating effect.

A bidentate (or) a poly dentate ligand is known as chelating ligand if on coordination it results in the formation of a closed (or) cyclic structure.Chelating ligands form more stable complexes than the monodentate ligands. This is called chelating effect.

The chelate effect describes the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal.

Consider the two equilibria, in aqueous solution, between the copper(II) ion, Cu²⁺ and ethylenediamine (en) on the one hand and methylamine, MeNH₂ on the other.

(2)²⁺]₂)₂ [Cu(MeNH ₂ + 2 MeNH²⁺Cu (1)²⁺ [Cu(en)] + en ²⁺Cu

In (1) the bidentate ligand ethylenediamine forms a chelate complex with the copper ion. Chelation results in the formation of a five-membered CuC₂N₂ ring. In (2) the bidentate ligand is replaced by two monodentate methylamine ligands of approximately the same donor power, meaning that the enthalpy of formation of Cu—N bonds is approximately the same in the two reactions.

The thermodynamic approach to describing the chelate effect considers the equilibrium constant for the reaction: the larger the equilibrium constant, the higher the concentration of the complex.

²]₂[Cu][MeNH₁₂]= β₂)₂[Cu(MeNH [Cu][en]₁₁[Cu(en)] =β

Electrical charges have been omitted for simplicity of notation. The square brackets indicate concentration, and the subscripts to the stability constants, β, indicate the stoichiometry of the complex. When the analytical concentration of methylamine is twice that of ethylenediamine and the concentration of copper is the same in both reactions, the concentration [Cu(en)] is much higher than the concentration [Cu(MeNH₂)₂] because β₁₁ >> β₁₂.

An equilibrium constant, K, is related to the standard Gibbs free energy, ΔG by

A bidentate (or) a poly dentate ligand is known as chelating ligand if on coordination it results in the formation of a closed (or) cyclic structure.Chelating ligands form more stable complexes than the monodentate ligands. This is called chelating effect.