Direct Observation of the Intermediates in the Photochemical Reaction of R₃SiCo(CO)₄ with R′₃SiH and Establishment of the Mechanism for R′₃SiCo(CO)₄ Formation

Irradiation of R₃SiCo(CO)₄ in the presence of R′₃SiH yields R′₃SiCo(CO)₄ and R₃SiH at 298 K in an alkane solvent. The mechanism for this reaction has been established by irradiation at low temperature in alkane media. At 77 K in a rigid alkane medium, irradiation of R₃SiCo(CO)₄ (R = Me, Et, Ph) yields dissociative loss of CO to give a 16 e⁻ complex, R₃SiCo(CO)₃, as determined by infrared spectroscopy. If the matrix contains a sufficiently high concentration of R₃SiH (R = Et) the light-induced loss of CO occurs, but the metal carbonyl product is (R₃Si)(R′₃Si)Co(CO)₃H. This species can also be formed photochemically at 196 K in fluid solution. The (Et₃Si)₂Co(CO)₃H is sufficiently inert that the Co-hydride resonance in the ¹H NMR can be observed at δ −9.8. Warmup of the (R₃Si)(R′₃Si)Co(CO)₃H to 298 K results in generation of both R₃SiCo(CO)₄ and R′₃SiCo(CO)₄. The low-temperature photochemistry thus provides direct evidence for both the 16 e⁻ primary photoproduct and the Co(III) oxidative addition product in the exchange mechanism.