Infrared spectra of M-η2-C2H2 and HM-C≡CH produced in reactions of laser-ablated group 6 metal atoms with acetylene.

The π and C-H insertion complexes (M-η(2)-C(2)H(2) and HM-C≡CH) are identified in the matrix infrared spectra from reactions of laser-ablated Group 6 metal atoms with acetylene. In annealing, the π complex is produced, and it converts to the insertion product during photolysis with no trace of the vinylidene product. This observation is consistent with the considerably higher activation energy to H(2)CCM than that to HM-CCH in the previously proposed reaction path, whereas the three plausible products are in fact energetically comparable. The back-donations in the Group 6 metal π complexes are evidently weaker than those in the Groups 3-5 metal analogues but still stronger than those in the main group and Group 7-10 metal systems. The insertion complexes have bent CMH moieties in contrast with the linear Mn complex.