Complexation Equilibria: The Chelate Effect

Recall that in complexation, metal ions react with ligands to form complexes.

These ligands can be either monodentate with one donor site or polydentate with more than one donor center.

Polydentate ligands, when reacting with metal cations, form complexes that resemble the claws of a crab holding an object. Because of this, the ligands are called chelating agents and their complexes are chelates.

Chelating ligands form more stable complexes with metal ions than a similar monodentate ligand. This can be understood from its thermodynamics.

For a complexation reaction, the enthalpy change is almost the same when ligands having similar groups are involved. However, the disorder or change in entropy varies.

In the case of complexes formed by monodentate ligands, the disorder of the reaction remains the same.

However, polydentate ligands increase the disorder of complex formation by increasing the entropy of the reaction.

Because the formation of chelates is entropy favored, it is termed the entropy or chelate effect.