Hard and Soft Acids and Bases (HSAB) Principle
Hard Soft Acids Bases (HSAB) Principle
Pearson Theory
Hard Soft Acids Bases (HSAB) Principle is a qualitative concept or prediction introduced by Ralph Pearson to explain the stability of metal complexes, mechanisms of their reactions and their pathways.According to the HSAB principle, hard acid prefers to combine with a hard base and soft acid prefers to combine with a soft base in order to form a stable acid-base complex. Hard acids interact strongly with a hard base to form stable ionic complexes, while soft acids interact strongly with a soft base to form covalent complexes.
Hard Acid + Hard Base → Stable Complex
Soft Acid + Soft Base → Stable Complex
Hard Acid + Soft Base → Less Stable Complex
Soft Acid + Hard Base → Less Stable Complex
Strong ionic interaction results from a large electronegativity difference between hard acid and hard base, while the interaction between soft acid and soft base leads to strong covalent interaction. However, the interaction between hard acid and soft base or soft acid and hard base results in polar covalent interactions which are more reactive and less stable. This is the reason that hard acid prefers to combine with hard acid or soft acid prefers to combine with a soft base in order to form more stable complexes.
Hard Lewis Acids
Hard 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
Soft Lewis acids are characterized by large ionic radii, low positive charge, completely filled atomic orbitals and with low energy LUMOs.Hard Lewis bases
Hard Lewis bases are characterized by small ionic radii, strongly solvated, highly electronegative, weakly polarizable and with high energy HOMOs.Soft Lewis Bases
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.
NOTE:
☛ It is not necessary for Lewis acid or base to possess all the properties to be classified as hard or soft or borderline.
☛ Hard acids and bases are small and non-polarizable, whereas Soft acids and bases are larger and more polarizable.
☛ 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.
Limitation of HSAB Concept /Principle
Hard and soft classification is useful concept no doubt but it has some tricky limitations which are given below-1. The main limitation of the HSAB concept is that it is widely general and has no any direct quantitative scale of acid base strength.
2. The inherent acid base strengths are not accounted for e.g. OH- and F- ions are both hard bases where OH- is nearly 1013 times stronger base than F- ions. Correlation between hardness and inherent acid base strength is yet to be developed.
3. Sometimes Hard Soft principal fails to keep parity with inherent acid-base strengths. e.g.
CH3(g) + H2(g) → CH4(g) + H+ (g)
This reaction must be favoured in the view of soft-soft combination between CH3 and H-.
But in actual practice the combination is endothermic by about +360 kJ/mol. This unfavourable entropy change dose not allow the reaction to proceed.
