The structure and energy of SiH+5. comparisons with CH+5 and BH5

Differences between SiH⁺₅ and CH⁺₅ are more significant than the similarities. The proton affinity of SiH₄ exceeds than of CH₄ by ≈25 kcal/mol. but the heat of hydrogenation of SiH⁺₃ is smaller than that of CH⁺₃ by nearly the same amount. Like CH⁺₅ the C₅ structures of SiH⁺₅ are preferred, but SiH⁺₅ is best regarded as a weaker SiH⁺₃—H₂ complex. D_3h, C_2v, and C_4v forms are much higher in energy and SiH⁺₅ should not undergo hydrogen scrambling (pseudorotation) readily, as does CH⁺₅ The neutral BH₅ is only weakly bound toward loss H₂, and the D_3h. C_2v, and C_4v forms are also high in energy. The contral-atom electronegativities, C⁺ > B > Si⁺, control this behavior. The electronegativities also determine the ability to bear positive charges. Thermodynamically. SiH⁺₅ and SiH⁺₃ are more stable than CH⁺₅ and CH⁺₃, respectively; hydride transfer occurs from SiH₄ to CH⁺₃ and proton transfer from CH⁺₅ to SiH₄.