Computational and experimental studies on the mechanism of the photochemical carbonylation of group 6 Fischer carbene complexes.

The photocarbonylation reaction of Group 6 Fischer carbene complexes has been studied by DFT and experimental procedures. The process occurs by intersystem crossing (ISC) from the lowest excited singlet state (S1) to the lowest triplet state (T1), the latter structure being decisive for the outcome of the reaction. Methylenepentacarbonylchromium(0) complexes, alkoxypentacarbonylchromium(0)carbene complexes, and alkoxyphosphinetetracarbonylchromium(0) carbene complexes have coordinatively unsaturated chromacyclopropanone T1 structures with a biradical character. The evolution of the metallacyclopropanone species occurs by a jump (spin inversion) to the S(0) hypersurface by coordination of a molecule of the solvent, leading to ketene-derived products in the presence of ketenophiles or reverting to the starting carbene complex in their absence. The T1 excited states obtained from methylenephosphinetetracarbonylchromium(0) complexes and pentacarbonyltungsten(0)carbene complexes are unable to produce the carbonylation. The reaction with ketenophiles is favored in coordinating solvents, which has been tested experimentally in the reaction of alkoxypentacarbonylchromium(0) complexes and imines.