What is the Hard Acid Soft Base (HSAB) theory?

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Detailed Solution

The HSAB theory explains stability of chemical complexes: hard acids prefer hard bases, and soft acids prefer soft bases.

Proposed by Pearson, it classifies acids and bases by polarizability and charge density.

Type	Nature	Examples
Hard acids	Small, high charge, less polarizable	H⁺, Na⁺, Ca²⁺, Al³⁺
Soft acids	Large, low charge, easily polarizable	Ag⁺, Hg²⁺, Pt²⁺
Hard bases	Small atoms, high electronegativity, low polarizability	F⁻, OH⁻, NH₃, H₂O
Soft bases	Large, easily polarizable	I⁻, S²⁻, PPh₃, CN⁻

Hard–hard and soft–soft interactions form stable compounds (e.g., AgI, CaF₂). Helps predict reaction direction and selectivity in coordination chemistry.

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