Free-energy Relationships in Coordination Chemistry. III. A Comprehensive Index to Complex Stability

Efforts to relate metal-complex stability constants to some property of the metal ions involved have led to the development of a correlating numerical index Q with the following characteristics: (i) Q is a two parameter quantity involving ionic charge and electronegativity, which may assume different values for a particular metal according to the type of ligand with which complexes are formed; (ii) linear relationships are obtained for plots of log K ML vs. Q for various metals and more than thirty ligands. The effect of the thermodynamic and structural properties of ligands and metal complexes on the slope and intercept in this correlation is discussed. Two new indices to class b behavior are introduced, one for metal ions and one for ligands. The index Q is shown to be related to other published indices of ionic or covalent, class a or class b, behavior of metals and ligands; the alternative ways of assigning Q may be regarded as reflecting the varying degrees of ionic or covalent character in the complexes of a given metal.