TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 7 Issue 5 (2002) pp 261-264

The Three-Center, Four-Electron Bond in Hexacoordinated AB₆-Type Main Group Molecules: An Alternative Model of Bonding without d-Orbital Participation in the Central Atom

Xiaoping Sun

Department of Natural Science and Mathematics, University of Charleston, 2300 MacCorkle Avenue S. E., Charleston, West Virginia 25304, xsun@ucwv.edu
Received June 3, 2002. Accepted August 16, 2002

Published online: 13 September 2002

Abstract. 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^– (O_h 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

Key Words:  In the Classroom; inorganic chemistry; point groups

(*) Corresponding author. (E-mail: xsun@ucwv.edu)

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Issue date: October 1, 2002

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