Evaluation of softness from the stability constants of metal-ion complexes

Utilizing the dual parameter equation we derived previously, we have evaluated quantitatively softness or class (b) character of metal ions on the basis of the original criteria of soft-hard classification, viz., the stability of metal-ion complexes. Various softness parameters proposed so forth have been proved insufficient to describe softness quantitatively, although they are useful for a rough classification of metal ions into soft and hard acids. Discussion is also given concerning origin of softness, especially from the viewpoint of its closs correlation with the solvation effect.