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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: , alkali ions, , , , ...
Soft acids: , , , , , , , ...
Hard bases: , , (), , , ...
Soft bases: , , , ...
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 + +
Where (Methylmercury) is a very soft acid an (proton) is a hard acid, which compete for the B (base to be classified).
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