What are Hard Soft Acids and Bases

Hard Soft Acids and Bases

What are Hard Soft Acids and Bases ?

Hard Acids

Hard acids are such acids in which the acceptor atoms have high oxidation states, small size and a 8 electron structure. They have vacant orbitals to accomodate electron pairs. These are generally the cations of group IA to IVA in higher oxidation states.
Examples-
H⁺, Li⁺, Na⁺, K⁺, Be⁺², Mg⁺², Ca⁺², Sr⁺², Sn⁺², Al⁺³, Ga⁺³, In⁺³, Cr⁺³, Co⁺³, Fe⁺³, Ir⁺³, La⁺³, Si⁺⁴, Ti⁺⁴, Zr⁺⁴, BF₃, AlCl₃, CO₂ etc.

Soft Acids

Soft acids are such acids in which the acceptor atoms have low oxidation states, large size and a 18 electron structure. These are generally the cations of group IB and IIB in lower oxidation states usually 0 or +1. They have 6 or more valence electrons and a d¹⁰ configuration.
Examples-
Cu⁺, Ag⁺, Au⁺, Hg⁺ , Cs⁺ , Tl+ , Hg⁺² , Pd⁺², Cd⁺² , Pt⁺², I₂ and Br₂ etc.

Hard Bases

Hard bases are such base in which the donar atoms have small size and high electronegativity. They have filled valence orbitals to donate a σ-pair or electrons.
Examples-
H₂O, OH-, F-, Cl-, CH₃CO₂^-, PO₄^3-, SO₄^2-, CO₃^2-, NO₃^-, ClO₄^-, ROH, RO^-, R₂O, NH₃, RNH₂, N₂H₄ etc.

Soft Bases

Soft bases are such base in which the donar atoms have large size and low electronegativity. π bonding ligands are generally soft bases.
Examples-
S^-2, RSH, RS^-, R₂S, I^-, CN^-, SCN^-, S₂O₃^-, R₃P, R₃As, (RO)₃P, RNC, CO, C₂H₄, C₆H₆, R^-, H^- etc.

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HSAB Principle | Pearson Theory

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