The HSAB concept, also known as HSAB theory, is widely used in chemistry for explaining stability of compounds, reaction paths etc. // Introduction Chemistry is a large field encompassing many subdisciplines that often overlap with significant portions of other sciences. ... A chemical compound is a chemical substance formed from two or more elements, with a fixed ratio determining the composition. ... A chemical reaction is a process involving one, two or more substances (called reactants), characterized by a chemical change and yielding one or more product(s) which are different from the reactants. ...

HSAB is the acronym for hard and soft acids and bases An acid (often represented by the generic formula AH) is typically a water-soluble, sour-tasting chemical compound. ... The common (Arrhenius) definition of a base, also known as an alkaline compound, is a chemical compound that either donates hydroxide ions or absorbs hydrogen ions when dissolved in water. ...

It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. In biology, the most commonly used definition of species was first coined by Ernst Mayr. ...

'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, ... The alkali metals are the series of elements in Group 1 (IUPAC style) of the periodic table, excluding hydrogen: that is, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). ...

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

Hard bases: OH^–, F^–, Cl^–, NH₃, CH₃COO^–, CO₃^2–, ...

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). Electron configurations of lithium and fluorine. ... Covalently bonded hydrogen and carbon in a molecule of methane. ...

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).