MECHANISM OF THE TA-MEDIATED ACTIVATION OF THE C-H BOND IN METHANE()

The mechanistic details of the C-H bond activation brought about by a Ta+ ion, which was observed in the gas phase, have been computationall y investigated using approximate density functional theory, Hn accord with the experimental information, the overall reaction sequence Ta+ CH4 --> TaCH2+ + H-2 is compute to be exothermic by ca. 14 kca mol(-1 ) and the Ta-C binding energy in the tantalum carbene cation is comput ed as 119 kcal mol(-1), in satisfying agreement with the experimental lower bound for this quantity of III kcal mol(-1), The bond activation process itself consists of several steps, commencing with the formati on of an electrostatically bound encounter complex,fallowed by the ins ert-ion of the Ta+ into a C-H bond and the generation of a H2TaCH2+ di hydrido species, Finally, this latter intermediate rearranges into a p roductlike complex between molecular hydrogen and a tantalum carbene c ation, which concludes the sequence: by subsequent loss of H-2. Intere stingly, the reaction does not proceed adiabatically on one potential energy surface; rather, there are several changes in multiplicity alon g the reaction coordinate. The structures of singlet TaCH2+ and relate d intermediates where a carbene substructure is involved show several peculiarities, which are rationalized by an analysis of the relevant m olecular orbital and the electron density.