Question

xplain why chelating ligands give more stable complexes than monodentate ligands; use a balanced chemical equation in your explanation. Explain why macrocyclic ligands give more stable complexes than chelating ligands.

Answer 1

Chelate ligands give more complexes than monodentate ligands. The stable complex formation with chelate ligand is driven by entropy change.

Consider the equilibrium reaction for a M²⁺ metal ion.

[M(H₂O)₆] ²⁺ + 6NH₃ ---------> [M(NH₃)₆] ²⁺ + 6H₂O                  

[M(H₂O)₆] ²⁺ + 3en ---------> [M(en)₃] ²⁺ + 6H₂O     [ethylenediamine (en)]

In first reaction, there is no change in number of molecule before and after reaction. So the ΔS = 0.

In the next reaction, ΔS = +Ve (4 reactant molecule gives 7 product molecules).

In both complex total number of binding site and the bond strength of M-N is same. So the change in enthalpy for both complex is almost the same.

The reaction free energy, ΔG = ΔH – TΔS = -RTlnK

At a constant temperature, a chelating ligand will give the more +Ve value of ΔS. Which favors the -Ve ΔG value. So the stable complex formation occurs spontaneously.

Macrocyclic ligands give more stable complexes than chelate ligands. An n-dented macrocyclic (chelating) ligand mainly forms a more stable complex compared to an n-dented linear chelating ligand. This is known as the macrocyclic effect. This effect comes from a very favorable entropy change (∆S) including a favorable enthalpy (∆H) change. Due to this macrocyclic ligands give more stable complex.

Stability of metal complex with different ligand: Macrocyclic > Chelate > Monodentate