The electronic structure and photochemistry of group 6 bimetallic (Fischer) carbene complexes: beyond the photocarbonylation reaction.

The UV spectra of Group 6 metal carbene complexes bearing a CpM(CO)(3) (Cp = cyclopentadienyl) moiety bonded to the carbene carbon atom exhibit a redshift of the absorption maxima at higher wavelengths with respect to the parent monometallic complexes. This redshift is partly due to a higher occupation on the p(z) atomic orbital of the carbene carbon atom. Time-dependent DFT calculations accurately assign this band to a metal-to-ligand charge-transfer transition, thus showing that the presence of a second metal center does not affect the nature of the transition. However, the photochemical reactivity of Group 6 metal carbene complexes bearing a CpM(CO)(3) moiety strongly depends on the nature of this metal fragment. A new photoslippage reaction leading to fulvenes occurs when Mn-derived products 11 a, 11 b, and 12 a are irradiated (both Cr and W derivatives), whereas Re-derived product 11 c behaves like standard Fischer complexes and yields the usual photocarbonylation products. A new photoreduction process occurring in the metallacyclopropanone intermediate is also observed for these complexes. Both computational and deuteration experiments support this unprecedented photoslippage process. The key to this differential photoreactivity seems to be the M-Cp back-donation, which hampers the slippage process for Re derivatives and favors the carbonylation reaction.