New Insights into the Remarkable Difference between CH5- and SiH5.

It has long been known that there is a fundamental difference in the electronic structures of CH5- and SiH5-, two isoelectronic molecules. The former is a saddle point for the SN2 exchange reaction H- + CH4 → [CH5-]‡ → CH4 + H-, while the latter is a stable molecule that is bound relative to SiH4 + H-. SCGVB calculations indicate that this difference is the result of a dramatic difference in the nature of the axial electron pairs in these anions. In SiH5-, the axial pairs represent two stable bonds-a weak recoupled pair bond dyad. In CH5-, the axial electron pairs represent an intermediate transition between the electron pairs in the reactants and those in the products. Furthermore, the axial orbitals at the saddle point in CH5- are highly overlapping, giving rise to strong Pauli repulsion and a high barrier for the SN2 exchange reaction.