2.15:
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.
2.15:
Complexation Equilibria: The Chelate Effect
In complexation reactions, metal atoms or cations interact with ligands to form donor-acceptor adducts called metal complexes. Ligands that bind through one donor site are monodentate, ligands with two donor sites are bidentate, and those with more than two donor sites are polydentate ligands. For example, ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms, forming a five-membered ring. EDTA is a polydentate ligand that binds through four oxygen and two nitrogen atoms.
Bidentate and polydentate ligands are also called chelating agents, and the corresponding complexes are called chelates. Chelate is a Greek word that means "claw-like." Metal complexes formed by the chelating agents are more stable than those formed by their monodentate counterparts, as the reaction for their formation is entropically favored. This property is known as the chelate effect or the entropy effect.