Pearson principle of hard and soft acid and base

Pearson's Principle or SHAB Principle

Pearson suggests a rule for the prediction of the stability of complexes formed between acids and bases.

Hard acids prefer to combine with hard bases and soft acids prefer to combine with soft bases.

If a complex AB in which both A and B are both either hard acid and base or soft acid and base then AB complex will be the most stable. In this way, the Pearson concept provides the basis for the prediction of the approximate relative stability of complexes.

 

 

Due to the presence of large electronegativity difference between hard acid and hard base give rise to strong ionic attraction and the electronegativity of soft acid and soft base are almost the same therefore interaction between them is more covalent than ionic.