WHY IS SIH5- A STABLE INTERMEDIATE WHILE CH5- IS A TRANSITION-STATE - A QUANTITATIVE CURVE CROSSING VALENCE BOND STUDY

Valence bond computations of curve-crossing diagrams reveal a fundamental difference between the title species. The stability of SiH5- does not derive from hypervalency associated with d-AOs on Si but rather from the ability of Si to utilize its Si-H σ* orbitals for bonding, much more so than C does with its σ*(C-H) orbitals. Consequently, SiH5- possesses two resonating H--Si--H axial bonds; one via the axial p-AO of Si and the other via the equatorial σ*(SiH3) orbital of the central SiH3 fragment. As a result of the bonding capability oσ*(SiH3), SiH5- can delocalize efficiently the fifth valence-electron pair into the equatorial Si-H bonds. The energy of SiH5- is thus lowered by the delocalization relative to SiH4 + H- No significant stretching of the axial bonds is required to achieve this delocalized state, and therefore the bond lengths of SiHj- do not exceed those of SiH4 by much. On the other hand, the σ*(CH3) orbital possesses no bonding capability. The analogous delocalization of the fifth valence-electron pair is prohibited by the high promotion energy p → σ* and by the nearly zero overlap of σ*(CH3) with the axial hydrogens. As an alternative, CH5- localizes its fifth valence electron into the axial H--C--H linkage. This option leads to a long H--C--H linkage and a high energy of CH5- relative to CH4 + H-. © 1990, American Chemical Society. All rights reserved.