Pentacoordinated AB5-Type Main Group Molecules Favorably Adopt sp2 Hybridization in the Central Atom

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 → 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.