Pearson's HSAB Principle - Concept - Applications - Limitations - CSIR NET GATE AdiChemistry IIT JAM

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Pearson's Hard Soft Acids & Bases HSAB Principle - Concept-Applications
Hard and Soft Acids and Bases (HSAB) Principle is a qualitative concept introduced by Ralph Pearson to explain the stability of metal complexes and the mechanisms of their reactions. However, it is also possible to quantify this concept based on Klopman's FMO analysis using interactions between HOMO and LUMO. The principle, definitions, examples, theoretical basis, applications, and limitations of Pearson's HSAB theory are presented in this document.


According to the HSAB principle, the Lewis acids and bases can be further divided into hard or soft or borderline types.

hard acid and soft acid definition

Hard Lewis acids are characterized by small ionic radii, high positive charge, strongly solvated, empty orbitals in the valence shell and high energy LUMOs.

Soft Lewis acids are characterized by large ionic radii, low positive charge, completely filled atomic orbitals, and 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 Borderline Lewis acids and bases have intermediate properties.
HSAB theory by Ralph Pearson
HSAB Applications

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According to the 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. It is sometimes referred to as the Hard-Soft Interaction Principle (HSIP).

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