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.

Spin-Coupled Generalized Valence Bond Description of Group 14 Species: The Carbon, Silicon and Germanium Hydrides, XH n ( n = 1-4).

Although elements in the same group in the Periodic Table tend to behave similarly, the differences in the simplest Group 14 hydrides-XH n (X = C, Si, Ge; n = 1-4)-are as pronounced as their similarities. Spin-coupled generalized valence bond (SCGVB) as well as coupled cluster [CCSD(T)] calculations are reported for all of the molecules in the CH n/SiH n/GeH n series to gain insights into the factors underlying these differences. It is found that the relative weakness of the recoupled pair bonds of SiH and GeH gives rise to the observed differences in the ground state multiplicities, molecular structures, and bond energies of SiH n and GeH n. A number of factors that influence the strength of the recoupled pair bonds in CH, SiH, and GeH were examined. Two factors were identified as potential contributors to the decrease in the strengths of these bonds from CH to SiH and GeH: (i) a decrease in the overlap between the orbitals involved in the bond and (ii) an increase in Pauli repulsion between the electrons in the two lobe orbitals centered on the X atoms. Finally, an analysis of the hybridization of the SCGVB orbitals in XH4 indicates that they are closer to sp hybrids than sp3 hybrids, which implies that the underlying cause of the tetrahedral structure of the XH4 molecules is not a direct result of the hybridization of the X atom orbitals.