Hyperconjugative Effects in Carbenium and Silicenium Ions

Bond dissociation energy (R₃M⁺ - L) and bond length (R - M and M - L) trends in the R₃ML⁺ series of cationligand (L) complexes for M = carbon and silicon, and R = H, CH₃ and F are derived from density functional theory calculations using the hybrid B3LYP exchange-correlation potential. The ligands studied are NH₃, H₂O, HCN, H₂CO, MeCN, Me₂O, Me₂CO, FCN, F₂O, F₂CO, and NF₃, where ligand binding to M is through the nitrogen or oxygen atom. For all ligand substrates, R₃M⁺ - L bond energies are calculated to decrease from carbenium to silicenium with R = H but to increase for R=methyl and fluorine. Also for these latter two cases, in going from the bare R₃M⁺ cation to the ligand complexes, the R-M distances increase by more than twice as much for the carbenium than for the silicenium ions. These trends indicate the relative importance of a stabilizing R-M hyperconjugative interaction in the bare tert-butyl and trifluoromethyl cations compared with the other bare cations and all the cation-ligand complexes. Ab initio, multiconfiguration VBSCF calculations are carried out on model systems (AH_n - MH₂⁺; M = C, Si; AH_n = CH₃, SiH₃, F), designed to mimic the R₃M⁺ cations, in order to analyze the electronic structure of the R - M bond. The π bond component, representing the hyperconjugative interaction, is found to preferentially stabilize CH₃CH₂⁺ over SiH₃CH₂⁺, and FCH₂⁺ relative to FSiH₂⁺. The fluorosilicenium cation shows significant π donor effects. This analysis establishes the theoretical basis for the trends in energy and structural properties found for the R₃M⁺ cations and cation-ligand complexes.