Theoretical study on the stability of low-spin hydridomethyl complexes of the first-row transition metal cations

Theoretical calculations have been performed in order to study the stability of the low-spin hydridomethyl complexes HMCH(3)(+) for the first-row transition metals (M(+) = Sc+-Cu+). Originally experimental results have been rationalized by assuming a low-spin hydridomethyl complex as a stable intermediate in the reactions of methane with singly charged metal cations. Recently, theoretical studies showed that for some late transition metals of the first row (Fe+ and Co+) no stable low-spin insertion product could be located on the potential energy surface. For the early elements of this row (Sc+-V+) the experimental cross section ratios sigma(MH(+))/sigma(MCH(3)(+)) indicate that the elimination reactions for these cations proceed via a statistically behaved intermediate. Our CASPT2 calculations indeed confirm a stable hydridomethyl complex for these cations. The reason for the stability of the insertion complexes could be traced back to the relative position of the lowest lying low-spin s(0)d(n) state and the lowest lying low-spin s(1)d(n-1) state in the electronic spectrum of the corresponding free transition metal cations. Further, an analysis of the wave function clearly reveals a correlation between the extent of the participation of the 4s orbital in the metal-ligand bonds and the experimentally observed dominance of the H-2 elimination over the other elimination reactions for the cations Sc+ to Cr+. An explanation in terms of the frontier orbital approach is given.