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 SiHsdoes not derive from hypervalency associated with d-AOs on Si but rather from the ability of Si to utilize its Si-H u* orbitals for bonding, much more so than C does with its u*(C-H) orbitals. Consequently, SiHSpossesses two resonating H-Si-H axial bonds; one via the axial p A O of Si and the other via the equatorial u*(SiH3) orbital of the central SiH3 fragment. As a result of the bonding capability of u*(SiH3), SiHScan delocalize efficiently the fifth valence-electron pair into the equatorial Si-H bonds. The energy of SiH< 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 SiHSdo not exceed those of SiH4 by much. On the other hand, the u*(CH3) orbital possesses no bonding capability. The analogous delocalization of the fifth valence-electron pair is prohibited by the high promotion energy p u* and by the nearly zero overlap of o*(CH3) with the axial hydrogens. As an alternative, CHslocalizes 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 CHSrelative to CH, + H-. A fundamental chemical phenomenon is the different nature of the &$-type reactions at carbon and silicon.2 This difference is exemplified in the simplest of these reactions, the Hexchange in eqs 1 and 2.'