Unusually Slow Photodissociation of CO from (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>3</sub> (M = Cr or Mo): A Time-Resolved Infrared, Matrix Isolation, and DFT Investigation

The photochemistry of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)M(CO)<sub>3</sub> (M = Cr or Mo) is described. Photolysis with λ<sub>exc.</sub> > 300 nm of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>3</sub> in low-temperature matrixes containing CO produced the CO-loss product, while lower energy photolysis (λ<sub>exc.</sub> > 400 nm) produced Cr(CO)<sub>6</sub>. Pulsed photolysis (λ<sub>exc.</sub> = 400 nm) of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>3</sub> in <i>n</i>-heptane solution at room temperature produced an excited-state species (1966 and 1888 cm<sup>−1</sup>) that decays over 150 ps to (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>2</sub>(<i>n</i>-heptane) (70%) and (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>3</sub> (30%). Pulsed photolysis (λ<sub>exc.</sub> = 266 nm) of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>3</sub> in <i>n</i>-heptane produced bands assigned to (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>2</sub>(<i>n</i>-heptane) (1930 and 1870 cm<sup>−1</sup>) within 1 ps. These bands increase with a rate identical to the rate of decay of the excited-state species and the rate of recovery of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Cr(CO)<sub>3</sub>. Photolysis of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Mo(CO)<sub>3</sub> at 400 nm produced an excited-state species (1996 and 1898 cm<sup>−1</sup>) and traces of (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)Mo(CO)<sub>2</sub>(<i>n</i>-heptane) within 1 ps. For the chromium system CO-loss can occur following excitation at both 400 and 266 nm via an avoided crossing of a MACT (metal-to-arene charge transfer) and MCCT/LF (metal-to-carbonyl charge transfer/ligand field) states. This leads to an unusually slow CO-loss following excitation with 400 nm light. Rapid CO-loss is observed following 266 nm excitation because of direct population of the MCCT/LF state. The quantum yield for CO-loss in the chromium system decreases with increasing excitation energy because of the competing population of a high-energy unreactive MACT state. For the molydenum system CO-loss is a minor process for 400 nm excitation, and an unreactive MACT state is evident from the TRIR spectra. A higher quantum yield for CO-loss is observed following 266 nm excitation through both direct population of the MCCT/LF state and production of a vibrationally excited reactive MACT state. This results in the quantum yield for CO-loss increasing with increasing excitation energy.