hypervalent

Hypervalent is a term used in chemistry to describe species in which the central atom has more electrons in its valence shell than would be predicted by the octet rule, or forms more covalent bonds than eight electrons would allow. This expanded valence is particularly common for elements in the third period and heavier when bonded to highly electronegative ligands such as fluorine. Hypervalent molecules can be neutral, ionic, or polyatomic and often exhibit geometries near octahedral or with other expanded-shell arrangements.

Bonding in hypervalent species has been described in several ways. Historically, d-orbital participation in the central atom was invoked to accommodate extra electrons. Modern explanations often rely on three-center four-electron bonds and delocalized molecular orbitals, which account for expanded valence without requiring d-orbitals. In practice, hypervalent compounds may be described using valence bond theory, molecular orbital theory, or resonance structures, and many remain stable under ambient conditions.

Representative examples include sulfur hexafluoride (SF6), sulfur tetrafluoride (SF4), and phosphorus pentafluoride (PF5), as well as the hexafluorophosphate anion (PF6−). Xenon compounds such as xenon difluoride (XeF2) and xenon tetrafluoride (XeF4) illustrate that noble gases can form hypervalent species. Iodine pentafluoride (IF5) is another example. In organic chemistry, hypervalent iodine reagents (such as Dess–Martin periodinane and iodobenzene diacetates) are widely used as mild oxidants and enable a variety of selective transformations.