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Hard & Soft Acids and Bases

  1. R. Bruce Martin

Published Online: 15 MAR 2006

DOI: 10.1002/0470862106.ia269

Encyclopedia of Inorganic Chemistry

Encyclopedia of Inorganic Chemistry

How to Cite

Martin, R. B. 2006. Hard & Soft Acids and Bases. Encyclopedia of Inorganic Chemistry. .

Author Information

  1. University of Virginia, Charlottesville, VA, USA

Publication History

  1. Published Online: 15 MAR 2006


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.


  • hard and soft acids and bases