Why do soft acids/bases tend to have high polarizability?

Pearson, who developed HSAB theory, later found a molecular orbital basis for chemical hardness. Hardness ([math]\eta[/math]) is, by definition, related to the magnitude of the HOMO/LUMO gap. read more

Conversely, large gaps indicate a high energy for electronic delocalization and lower polarizability. This is why hard acids and bases tend to be small with fully occupied valence shells -- full valence stabilizes the atoms and results in a large HOMO/LUMO gap. read more

Rest all bases are called soft bases, because they have a lower tendency of releasing OH- ions. read more

This rule (established in 1954) predates HSAB theory but in HSAB terms its explanation is that in a S N 1 reaction the carbocation (a hard acid) reacts with a hard base (high electronegativity) and that in a S N 2 reaction tetravalent carbon (a soft acid) reacts with soft bases. read more