The Chelate Effect

Modern concepts of the nature of chelate compounds and interpretations of their high stability are considered in detail. The translational entropy contribution to stability, originally described by Schwarzenbach and Adamson, is broadened to include entropy increases resulting from displacement of solvated water molecules from cation and anion through charge neutralization effects. A favorable chelate enthalpy effect results from more positive heats of formation of chelating ligands. A detailed, general analysis of the constitutional factors controlling the stabilities of chelate compounds reveals five enthalpy effects and six entropy effects as significant. Principles are illustrated with thermodynamic data taken from the literature on metal chelates of polyamines and amino-polycarboxylic acids.