Chelation

The extra stability of a chelate relative to comparable monodentate complexes is largely entropic. Displacing several separate ligand molecules with one polydentate ligand increases the number of free particles in solution, and that positive entropy change lowers the Gibbs free energy of complex formation; the effect is known as the chelate effect. Ring size matters, five- and six-membered chelate rings being the most stable because they are least strained. EDTA is the standard illustration, offering six donor atoms and forming complexes of one-to-one stoichiometry with most metal ions, which is what makes it useful both in complexometric titration and as a sequestering agent in foods and detergents. Biological chelates include the porphyrin ring of haem and the magnesium complex at the centre of chlorophyll.