Hard and Soft Acids and Bases (HSAB) Theory is a qualitative concept introduced by Ralph Pearson to explain the stability of metal complexes and the mechanisms of their reactions.
However it is possible to quantify this concept based on Klopman' FMO analysis using interactions between *** and LUMO.
According to this theory, the Lewis acids and bases can be further divided into hard or soft or border line types.
In considering the acid-base equilibrium:
Lewis acid + Lewis base ⇌ acid-base productHard Lewis acids are characterized by small ionic radii, high positive charge, strongly solvated, empty orbitals in the valence shell and with high energy LUMOs.
Soft Lewis acids are characterized by large ionic radii, low positive charge, completely filled atomic orbitals and with low energy LUMOs.
Hard Lewis bases are characterized by small ionic radii, strongly solvated, highly electronegative, weakly polarizable and with high energy HOMOs.
Soft Lewis bases are characterized by large ionic radii, intermediate electronegativity, highly polarizable and with low energy HOMOs.
The Border line Lewis acids and bases have intermediate properties.
Remember that it is not necessary for Lewis acid or base to possess all the properties to be classified as hard or soft or borderline.
In short, Hard acids and bases are small and non-polarizable, whereas Soft acids and bases are larger and more polarizable.
*HSAB Principle:According to HSAB concept, hard acids prefer binding to the hard bases to give ionic complexes, whereas the soft acids prefer binding to soft bases to give covalent complexes.*
The large electronegativity differences between hard acids and hard bases give rise to strong ionic interactions.
The electronegativities of soft acids and soft bases are almost same and hence have less ionic interactions. i.e., the interactions between them are more covalent.
The interactions between hard acid - soft base or soft acid - hard base are mostly polar covalent and tend to be more reactive or less stable. The polar covalent compounds readily form either more ionic or more covalent compounds if they are allowed to react.
