Stationary points for reactions R'R"HX+ + YH --> [R'R"X-Y](+) + H-2 (I) and R'(CH3)HX+ + YH --> [R'HX-Y](+) + CH4 (II) (R', R" = CH3, H; X = C, Si; Y = CH3O, (CH3)(2)N, and C6H5) are located and optimized by the B3LYP/aug-cc-pVDZ method. A similar mechanism was found to be operative for both types of reactions with X = C and X = Si. Formation of the intermediate (adduct) results in the transfer of electron density from the electron-rich bases to the X atoms and in the growth of a positive charge on a hydrogen atom attached to Y. This mobile proton may shift from Y to X, and the relative energies of transition states for elimination reactions (DeltaE(0)(TS)) depend on the ability of the X atom to retain this proton. Therefore, DeltaE(0)(TS) grows on going from Si to C and with increasing numbers of methyl substituents. For X = C, the DeltaE(0)(TS) value for both reactions correlates well with the population of the valence orbitals of X in a wide range from -44 kcal/mol (methyl cation/benzene) to 31 kcal/mol (isopropyl cation/methanol). For X = Si this range is more narrow (from -19 to -5.0 kcal/mol), but all DeltaE(0)(TS) values are negative with the exclusion of silylium ion/benzene systems, adducts of which are pi-rather than a-complexes. The energy minima for product complexes for H2 elimination are very shallow, and several are dissociative. However, complexes with methane which exhibit bonding between X and the methane hydrogen are substantially stronger, especially for systems with X = Si. The latter association energy may reach 8 kcal/mol ((SiH)-H-... distance is 2Angstrom).