Ab initio HF and Cl calculations were performed to determine the equilibrium geometry of SiH−5 and SiH−3, the barrier for internal rotation (SiH−5) and inversion (SiH−3) and the stability of SiH−5 and further to study the effect of electron correlation on reaction energies. The gaussian-type basis included d and f functions on Si and a p set on II. The D3h structures of SiH−5 is lower in energy than the C4v structure by 2.9(3.2) kcal/mol (corresponding HF results in parentheses). SiH−3 has C3v structure, the inner-ion barrier computed is 26.2 (27.3) kcal/mol. SiH−5 turns out to be stable with respect to SiH4 + H− by 20.3 (13.8) kcal/mol, but it is unstable with respect to SiH−3 ← H2 by 6.3 (5.6) kcal/mol. These results show that electron correlation has a small effect on barriers of inversion (SiH−3) or pseudorotation (SiH−5), but may have a pronounced effect on reaction energies even if all systems involved have closed shells. The correlation energy contributions are analyzed in terms of intrapair and interpair terms in order to get a better understanding of the influence of correlation on reaction and activation energies.