Pentacoordinated AB5-Type Main Group Molecules Favorably Adopt sp2 Hybridization in the Central Atom: Bonding without d-Orbital Participation

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 7 Issue 1 (2002) pp 11-14

Pentacoordinated AB₅-Type Main Group Molecules Favorably Adopt sp² Hybridization in the Central Atom: Bonding without d-Orbital Participation

Xiaoping Sun

Department of Natural Science and Mathematics, University of Charleston, Charleston, West Virginia 25304
Received August 27, 2001. Accepted October 18, 2001

Published online: 21 December 2001

Abstract. The pentacoordinated AB₅-type main group molecules have long been thought to have sp³d hybridization of the central atom, accounting for their oxidation state and molecular geometry; however, this does not explain the difference in axial and equatorial bond distances within these molecules. In addition, d-orbital participation in bonding for nonmetals is energetically unfavorable because it requires a np to nd excitation energy. In this paper, we have proposed that the sp² hybrid orbitals are formed on the central atom, overlapping with ligand orbitals in the equatorial positions. The longer axial bonds are formed via the overlap of the unhybridized p orbital on the central atom and the two terminal ligand orbitals, resulting in a three-center, four-electron bond. Examining the partial charge on the central atom using relevant electronegativity values supports this model without a d component in bonding. The model explains well the structure and bonding in pentavalent molecules. We believe that the results will be useful in updating chemistry texts.

Key Words: In the Classroom; inorganic chemistry; general chemistry; VESPR; ligands; hybridization

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

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