Infrared spectra of HC≡C-MH and M-η2-(C2H2) produced in reactions of laser-ablated group 5 transition-metal atoms with acetylene.

The ethynyl metal hydride (HC≡C−MH) and metallacycle complexes (M-η2-(C2H2)) are identified in the matrix infrared spectra from reactions of laser-ablated group 5 metal atoms with acetylene. The observed intensity variations reveal spontaneous formation of the cyclic complex upon annealing and its conversion to the insertion product via oxidative C-H insertion reaction and also during subsequent photolysis. The less stable vinylidene complex is not identified. The high binding energies, low C-C stretching frequencies, and long C-C bonds of the group 5 metal M-η2-(C2H2) species all indicate strong bonding relative to the late-transition-metal and light-metal analogues, but they are somewhat weaker than the group 4 metal counterparts.