HYPERCOORDINATION IN GROUP-IV MH5 AND MH5- SYSTEMS

The energetics and bonding of the group IV hypervalent MH5 and MH5- D3h structures (M = C, Si, Ge, Sn) are examined in this paper. Ab initio all-electron calculations are used to predict the energies and geometries of the systems. The resulting electron densities are analyzed using the topological theory of atoms in molecules. It is found that the anion energetically is more stable than the neutral radical for M = Si, Ge, and Sn but not for M = C. Further distinguishing carbon from the other members of the group is the fact that the CH bond mostly is of covalent character while the other MH bonds are mostly of ionic character. Scrutinizing difference density maps and atomic property changes reveals that, upon anion formation, the incoming electron density becomes preferentially accumulated in the nonbonded regions of the axial and, to a lesser extent, equatorial hydrogens.