PearsonHSABPrinzip

Pearson HSAB principle, named after Ralph G. Pearson, is a qualitative framework in chemistry used to rationalize and predict the outcome of many acid–base reactions. It classifies acids and bases as hard or soft and posits that hard acids prefer to bind hard bases, while soft acids prefer soft bases; interactions between hard and soft partners tend to be less favorable.

Hard species are small, highly charged, and poorly polarizable; soft species are larger, less charged, and highly polarizable. Borderline species exhibit intermediate properties and can form bonds with either type. The principle explains trends in coordination chemistry, substitution reactions, and catalysis, helping chemists anticipate which ligands will stabilize a metal center or accelerate a substitution step.

Quantitative aspects were developed by extending the idea with global hardness and softness from density functional theory. Hardness measures resistance to changing electron density; softness is its inverse. These concepts provide qualitative guidelines and also support semi-quantitative comparisons of acid/base pairs.

Examples include hard acid–hard base pairs such as Mg2+ with F− or H2O, and soft acid–soft base pairs such as Pt2+ with phosphines or sulfide donors. Borderline interactions are those like Fe2+/Fe3+ with nitrogen or sulfur ligands.

Limitations include its status as a guideline rather than a universal law, with solvent effects, covalent character, and complex equilibria sometimes altering preferences. Since its introduction in 1963, the HSAB principle has become widely used in inorganic and organometallic chemistry, catalysis, and biochemistry, often guiding ligand design and interpretation of reactivity.