Mechanistic investigations of CO-photoextrusion and oxidative addition reactions of early transition-metal carbonyls: (η⁵-C₅H₅)M(CO)₄ (M = V, Nb, Ta)

The mechanisms for the photochemical Si–H bond activation reaction are studied theoretically using a model system of the group 5 organometallic compounds, η⁵-CpM(CO)₄ (M = V, Nb, and Ta), with the M06-2X method and the Def2-SVPD basis set. Three types of reaction pathways that lead to final insertion products are identified. The structures of the intersystem crossings, which play a central role in these photo-activation reactions, are determined. The intermediates and transitional structures in either the singlet or triplet states are also calculated to provide a mechanistic explanation of the reaction pathways. All of the potential energy surfaces for the group 5 η⁵-CpM(CO)₄ complexes are quite similar. In particular, the theoretical evidence suggests that after irradiation using light, η⁵-CpM(CO)₄ quickly loses one CO ligand to yield two tricarbonyls, in either the singlet or the triplet states. The triplet tricarbonyl 16-electron intermediates, ([η⁵-CpM(CO)₃]³), play a key role in the formation of the final oxidative addition product, η⁵-CpM(CO)₃(H)(SiMe₃). However, the singlet counterparts, ([η⁵-CpM(CO)₃]¹), play no role in the formation of the final product molecule, but their singlet metal centers interact weakly with solvent molecules ((Me₃)SiH) to produce alkyl-solvated organometallic complexes, which are observable experimentally. This theoretical evidence is in accordance with the available experimental observations.