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^-.