Chelate effect

Encyclopedia : C : CH : CHE : Chelate effect

The increased stability or inertness of a chelated complex is called the chelate effect. A chelating agent (or polydentate ligand) bonds to more than one coordination site on the central atom. Because it is necessary to break all of the bonds to the central atom for the ligand to be displaced, it requires more energy to increase the number of separate molecules. If a chelate were replaced by several monodentate ligands, the total number of molecules would decrease, whereas if several monodentate ligands were replaced by a chelate, the number of free molecules increases. The effect is therefore entropic in that more sites are used by less ligands and this leaves more unbonded molecules: a total increase in the number of molecules in solution and a corresponding increase in entropy.

It is related to the macrocyclic effect.

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