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 S_N2 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 S_N2 exchange reaction.