Mechanistic Study for the Photochemical Reactions of d⁶ M(CO)₅(CS) (M = Cr, Mo, and W) Complexes

The mechanisms of photoextrusion reactions are determined theoretically for the model system of six-coordinated M(CO)₅(CS) (M = Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD basis set. Three types of elimination reaction pathways (i.e., path I, path II, and path III for axial CO extrusion, equatorial CO extrusion, and CS ligand extrusion, respectively) are considered in this study. Theoretical findings show that the photoextrusion mechanism for Cr and Mo complexes proceeds as follows: M-S₀-Rea + hν → M-S₁-FC → M-CI → M-Pro + CO. This study shows that when the reactant, M(CO)₅(CS) (M-S₀-Rea), is photoirradiated by UV light, it is excited vertically to many low-lying singlet excited states. It then relaxes to the first singlet excited state from the Franck-Condon point (M-S₁-FC). After passing through a conical intersection point (M-CI), this species eliminates a CO group to yield a five-coordinated product, M(CO)₄(CS) (M-S₀-Pro). However, for the W analogue, the photolysis mechanism is represented as W-S₀-Rea + hν → W-T₁-Min → W-T₁-TS → W-T₁/S₀ → W-S₀-Pro + CO. That is to say, when the reactant, W(CO)₅(CS) (W-S₀-Rea), absorbs UV light, it is excited to its several low-lying excited states by a vertical excitation. This species may then return to an intermediate at the first triplet excited state (W-T₁-Min) by means of intersystem crossings or conical intersections. After passing through a triplet transition state (W-T₁-TS) and a subsequent intersystem crossing (W-T₁/S₀), this molecule finally loses a CO ligand to produce a photoproduct at the ground singlet state (W-S₀-Pro). In other words, conical intersections and intersystem crossings play a decisive role in these photoextrusion reactions for M(CO)₅(CS). Theoretical evidence from a kinetic viewpoint strongly supports the theory that the photolysis of M(CO)₅(CS) only produces CO-loss photoproducts rather than the CS-loss photoproduct. Theoretical analysis using the results of this study allows a good interpretation of the available experimental observations.