Hard and soft acids and bases

Hard and soft acids and bases (HSAB) theory classifies Lewis acids and bases by size, charge density, and polarizability. It predicts that hard acids tend to bind hard bases, while soft acids tend to bind soft bases.

Lewis acid: A chemical species that accepts an electron pair to form a bond. HSAB applies its hardness and softness categories to electron-pair acceptors.
Electron pair: Two electrons occupying the same orbital or participating together in a chemical bond. Lewis bases donate such pairs to acids.
Chelation: The binding of a metal ion by a ligand that forms multiple coordinate bonds to it. HSAB matching helps explain which donor atoms bind a metal center.
Ralph G. Pearson: An American chemist who developed and popularized the hard and soft acids and bases concept. Pearson formulated the framework and gave it its enduring name.
Chemical hardness: A measure of resistance to changes in an atom or molecule's electron population. It gives a quantitative concept related to the qualitative hard-soft labels.
Lewis base: A chemical species that donates an electron pair to form a bond. HSAB classifies the electron-pair donors that bind Lewis acids.
Coordinate covalent bond: A covalent bond in which both shared electrons are contributed by one atom. Many Lewis acid-base adducts form through this kind of donation.
Metal ion selectivity: Preferential binding or transport of one metal ion over others by a chemical system. Hard-soft matching can guide selective ligand design.
Lewis acid-base theory: A theory defining acids as electron-pair acceptors and bases as electron-pair donors. HSAB extends this broader framework by classifying its acids and bases.
Chemical softness: A measure of how readily an atom or molecule's electron population changes. It quantifies a property associated with soft acids and bases.

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