Photochemistry of CpMn(CO)3 and Related Derivatives: Spectroscopic Observation of Singlet and Triplet CpMn(CO)2

We report novel photochemistry derived from (eta 5-C5H3)Mn(CO)(3) (1a), (eta(5)-C5H4Me)Mn(CO)(3) (1b), (eta(5)-C5Me5)Mn(CO)(3) (1c), and (eta(5)-indenyl)Mn(CO)(3) (1d). Photolysis (>261 nm, 1 h) of the parent tricarbonyl (1a-d), matrix isolated in argon at 10 K, yields two species: the expected singlet dicarbonyl (1)(eta(5)-L)Mn(CO)(2) ((1)2a-d) and an additional compound assigned as the triplet dicarbonyl (3)(eta(5)-L)Mn(CO)(2) ((3)2a-d). Density functional theory calculations (B3LYP/LANL2DZ) support the structural assignments for (1)2, and (3)2. Natural bond orbital population analyses of (1)2a and (3)2a explain the source of the large coupling (Delta v(CO) 153 cm(-1)) between the carbonyl stretching vibrations in (3)2a. The triplet isomer ((3)2) is metastable, even at temperatures as low as 10 K We determined the rate constants for the thermal isomerization (3)2 -> (1)2 using dispersive kinetic analysis. As revealed by these rate constants, the triplet complexes display the following order of stability in this system: Ind >> Cp approximate to Cp' > Cp*. The spectroscopy and kinetics observed in various matrices (Ar, CH4, and Xe) do not differ appreciably. Experimental and computational results suggest that the singlet triplet energy gap (Delta E-ST) of CpMn(CO)(2) (2a) must be smaller than previous estimates.