What is hard acid base

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Area of ​​Expertise - General Chemi

The HSAB concept (also HSAB principle; engl.: "hard and soft acids and bases", German translation: concept of hard and soft acids and bases) was developed in 1963 by R. G. Pearson. It is a classification principle for Lewis acids and bases with special consideration of size, charge, electronegativity and polarizability:

Hard acids:
Lewis acids with low polarizability, i.e. small and highly charged cations as well as molecules in which an electronegative binding partner induces a high positive charge on the central atom (electrophilic center), e.g.,,.
Soft acids:
Lewis acids with high polarizability, i.e. cations with low charge and large radius or atoms and molecules with occupied outer orbitals (easily removable valence electrons), e.g.,.
Hard bases:
Lewis bases with high electronegativity and low polarizability, e.g.,,.
Soft bases:
Lewis bases with low electronegativity and greater polarizability, e.g.,.

According to Pearson, the following applies to the course of an acid-base reaction: hard acids react preferentially with hard bases, soft acids react more with soft bases.

The HSAB concept is only of qualitative character, but allows far-reaching assessments of the course of reactions, the position of equilibria and the stability of reaction products.

See also: Lewis acid, Lewis base

Learning units in which the term is dealt with

Acid-base theories40 min.

chemistryGeneral Chemistryacids and bases

This learning unit deals with the different acid-base theories.

Reactivity: hardness, polarizability, HSAB concept30 min.

chemistryTheoretical chemistryChemical reactivity

In this learning unit, HOMO and LUMO energies are presented as a quantitative measure of the reactivity of inorganic nucleophiles and electrophiles.