The Physical Organic Chemistry of Fischer Carbene Complexes

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)5MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes

The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)₅M=C(Me)(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Møller-Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play a decisive role in these photoisomerizations, are determined. The former is applied to the chromium and molybdenum systems because their photoproducts are essentially from the singlet excited states. The latter is applied to the tungsten complex because its photoproducts are formed from a low-lying triplet excited state. Two reaction pathways are examined in this work: photocarbonylation (path I) and CO-photoextrusion (path II). The model studies strongly indicate that in the photochemistry of Cr and Mo Fischer carbene systems, the formation of metallaketene intermediates may occur at higher excitation wavenumbers, whereas the five-coordinated complexes that are attached by a solvent molecule are obtained at lower excitation wavenumbers. However, in the W analogue, because the activation barriers for path I are greater than that for path II and path I has more reaction steps than path II, the quantum yields for the metallaketene intermediate should be smaller than those for the five-coordinated species, which is also attached by a solvent molecule. These theoretical studies also suggest that the conical intersection and the spin crossover mechanisms that are identified in this work explain the process well and support the experimental observations.