A model study on the photodecarbonyl reaction of (η⁵-C₅H₅)M(CO)₂ (M = Co, Rh, Ir)

The group 9 organometallic complexes η⁵-CpM(CO)₂ (M = Co, Rh, and Ir) and Si(CH₃)₃(H) have been considered as a model system to study their photochemical decarbonyl reactions as well as the Si-H bond activation reactions using the CASSCF and MP2-CAS computational methods. For the cobalt complex, three kinds of reaction pathways, which result in the same oxidative addition product, are investigated. Our theoretical finding demonstrated that after the photoirradiation, η⁵-CpCo(CO)₂ loses one CO ligand without any difficulty to form either the triplet ([η⁵-CpCo(CO)]³) or singlet ([η⁵-CpCo(CO)]¹) species. The former plays a decisive role in the formation of the final oxidative addition product. On the other hand, the latter plays no role in the production of the final product molecule, but its singlet cobalt center interacts weakly with solvent molecules ((Me₃)SiH) to produce an alkyl-solvated organometallic complex, which is experimentally detectable. The present works reveal that both η⁵-CpRh(CO)₂ and η⁵-CpIr(CO)₂ should adopt the conical intersection mechanism after they are irradiated by light. Moreover, our theoretical examinations strongly suggest that for the 16-electron monocarbonyl η⁵-CpM(CO) (M = Rh and Ir) species, the insertion into a Si-H bond by the Ir system is much more facile and more exothermic than that for the Rh counterpart.