HSAB

Hard and Soft Acids and Bases (HSAB) is a concept in chemistry introduced by Ralph Pearson in 1963 that classifies acids and bases according to their polarizability and charge density. The central idea is that hard species are small, highly charged, and poorly polarizable, whereas soft species are larger, have lower charge density, and are easily polarizable. Borderline species fall between these extremes.

According to HSAB theory, hard-hard and soft-soft interactions are more favorable than hard-soft interactions, and this qualitative preference helps predict the outcome of acid-base reactions, formation of metal complexes, and catalytic processes. The assignment of hardness is often qualitative: common hard acids include H+, Na+, Mg2+, Ca2+, Al3+ and hard bases include F-, OH-, H2O; soft acids include Ag+, Hg2+, Pt2+, and soft bases include I-, RS-, PR3 ligands, CO, and phosphines.

Beyond qualitative ideas, HSAB has a quantitative formulation using global hardness η and electronegativity χ; the chemical potential μ = -χ. Matching acid and base with similar hardness tends to stabilize the interaction, while mismatches may lead to weaker bonding or preferential binding of softer bases to softer acids.

Applications span inorganic and organometallic chemistry, such as predicting metal-ligand preferences in coordination complexes, guiding selectivity in catalysis, and rationalizing nucleophile-electrophile interactions in organic reactions. Limitations include the approximate, empirical nature of the concept and exceptions arising from solvent effects and complex electronic environments.