also known as HSAB theory

HSAB is the acronym for hard and soft acids and bases

The HSAB concept is widely used in chemistry for explaining stability of compounds, reaction paths etc. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species.

'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are well polarizeable.

Among the hard acids are: H⁺, alkali ions, Ti⁴⁺, Cr³⁺, Cr⁶⁺, BF₃, ...

Soft acids: CH₃Hg⁺, Pt⁴⁺, Pd²⁺, Ag⁺, Hg²⁺, Hg₂²⁺, Cd²⁺, BH₃, ...

Hard bases: OH^–, F^–, (Cl^–), NH₃, CH₃COO^–, ...

Soft bases: H^–, R₃P, SCN^–, I^–...

Generally spoken, acids and bases interact and the most stable interactions are hard-hard (ionogenic character) and soft-soft (covalent character).

An attempt to quantify the 'softness' of a base consists in determining the equilibrium constant for the following equilibrium:

BH + CH₃Hg⁺ H⁺ + CH₃HgB

Where CH₃Hg⁺ (Methylmercury) is a very soft acid and H⁺ (proton) is a hard acid, which compete for the B (base to be classified).