Pentavalents

Pentavalents are atoms or ions in which the central atom forms five covalent bonds or exhibits a formal oxidation state of +5. In inorganic chemistry, pentavalence is most often discussed for the pnictogens (group 15 elements), especially phosphorus, arsenic, antimony, and bismuth, where +5 is a common oxidation state. Prominent pentavalent compounds include phosphorus pentachloride (PCl5), phosphorus pentafluoride (PF5), and antimony pentafluoride (SbF5). Phosphorus in its oxide form is often described as P4O10, the anhydride of phosphoric acid, reflecting a P(V) center in a condensed oxide framework.

Five-coordinate centers can adopt geometric shapes such as trigonal bipyramidal or square pyramidal arrangements. For example, PF5 and AsF5 are typically described as trigonal bipyramidal. In some species, Berry pseudorotation allows rapid interconversion between equivalent five-coordinate structures, effectively exchanging axial and equatorial positions.

Bonding in pentavalent species has been a central topic in main-group chemistry. Early explanations invoked expansion of the octet and participation of d orbitals, but modern descriptions emphasize hypervalent bonding through three-center four-electron interactions and molecular orbital concepts. These views account for the stability of pentavalent centers without requiring accessible d orbitals on lighter elements.

Biologically and practically, pentavalent phosphorus is central to energy transfer and information storage in biology through phosphate groups, which involve phosphorus in the +5 oxidation state. Pentavalent phosphorus compounds also serve as key intermediates in synthesis and catalysis, reflecting the enduring importance of pentavalence in chemistry.