Hard & Soft Acids and Bases

Practical metal ion hardness–softness scales applicable to aqueous solutions and corresponding to what most chemists probably imply when they use the terms hardness and softness are presented. The scales are derived from experimentally determined log stability constant differences for substitution reactions of one ligand for another on a metal ion. Correlations between scales for different ligand systems are high but not perfect. In an analogous ligand scale, all oxygen donors are hard and all nitrogen donors borderline. The principle of hard and soft acids and bases finds its most consistent application, not in direct stability constant comparisons, but rather in log stability constant differences of appropriate substitution reactions. Keywords: hard and soft acids and bases