HSAB theory

HSAB theory predicts that hard Lewis acids preferentially bind hard Lewis bases, while soft acids preferentially bind soft bases. Hardness and softness describe how readily species deform their electron distributions.

Lewis acid: An electron-pair acceptor that forms a bond by accepting a pair of electrons from a donor. HSAB classifies Lewis acids by how concentrated or deformable their charge is.
Coordination chemistry: The study of compounds in which central atoms or ions bind surrounding molecules or ions called ligands. HSAB predicts which ligands preferentially bind particular metal centers.
Ralph G. Pearson: An American chemist who developed and popularized the hard and soft acids and bases principle. Pearson introduced the terminology and formalized the principle.
Drago–Wayland equation: An empirical equation that estimates Lewis acid–base interaction enthalpies using separate electrostatic and covalent parameters. It offers quantitative interaction estimates where HSAB gives a broad qualitative preference.
Lewis base: An electron-pair donor that forms a bond by donating a pair of electrons to an acceptor. HSAB classifies Lewis bases by the concentration and deformability of their electron density.
Metal–ligand bond: A chemical bond between a metal center and a ligand that donates electron density to it. Hard–soft matching provides a qualitative guide to metal–ligand bond preferences.
Lewis acid–base theory: A theory defining acids as electron-pair acceptors and bases as electron-pair donors. HSAB refines Lewis acid–base chemistry by distinguishing species' hardness and softness.
Frontier molecular orbital theory: A framework explaining reactions through interactions between the highest occupied and lowest unoccupied molecular orbitals. Orbital energies and overlap can explain selectivity beyond HSAB's hard–soft labels.
Electronegativity: A measure of an atom's tendency to attract shared electrons in a chemical bond. Electronegativity helps describe charge concentration, one aspect of chemical hardness.
Chelation: Binding of a metal ion by a ligand that attaches through two or more donor atoms. HSAB preferences can help explain which donor atoms a chelating ligand uses.

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