The Three-Center, Four-Electron Bond in Hexacoordinated AB6-Type Main Group Molecules

The hexacoordinated AB6-type main group molecules have long been thought to have sp3d2 hybridization on the central atom, accounting for their molecular geometry (octahedral). However, the s-p-d hybridization does not explain how an energetically unfavorable np to nd excitation in an atom of nonmetallic elements, such as sulfur and phosphorus, can be achieved. In this article, the author has re-examined bonding in SF6 and PF6– (Oh symmetry) and proposed that the linear F–S–F and F–P–F bonds in both species are formed via the overlap of the 3p orbital on the central atom with terminal ligand orbitals, resulting in a three-center, four-electron bond. This alternative model, which does not involve d orbitals in bonding, is supported by a partial charge analysis using Allen’s electronegativity approach. SF6 or PF6– can be characterized by several ionic resonance structures containing a postulated SF42+ or PF4+ cation (octet on sulfur or phosphorus). The three-center, four-electron bond model can also be used to study bonding in hexacoordinated AB5E (e.g., halogen pentafluorides) and AB4E2 (e.g., xenon tetrafluoride) explaining well the molecular geometry. The author believes that all the results will be useful in updating chemistry texts