The ability of an atom in a molecular entity to expand its valence shell beyond the limits of the Lewis octet rule. Hypervalent compounds are common for the second and subsequent row elements in groups 15ā€“18 of the periodic table. A description of the hypervalent bonding implies a transfer of the electrons from the central (hypervalent) atom to the nonbonding molecular orbitals which it forms with (usually more electronegative) ligands. A typical example of the hypervalent bond is a linear three-centre, four-electron bond, e.g. that of Fapā€“Pā€“Fap fragment of PF5.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1946 (https://doi.org/10.1351/pac199971101919)