PHOTOCHEMISTRY OF (ETA(6)-2,6-X(2)C(5)H(3)N)CR(CO)(3) (X=H, CH3, (CH3)(3)SI) - FIRST EXAMPLE OF A PHOTOINDUCED RING-SLIP AT AN (ETA(6)-ARENE)M(CO)(3) CENTER - MOLECULAR-STRUCTURES OF (ETA(6)-2,6-(CH3)(2)C5H3N)CR(CO)(3) AND (ETA(6)-2,6-((CH3)(3)SI)(2)C5H3N)CR(CO)(3)

The photochemistry of (eta(6)-2,6 .. X(2)C(5)H(3)N)Cr(CO)(3) was inves tigated both in low-temperature matrices (X = H or (CH3)(3)Si) and in room-temperature solution (X = H, CH3, or (CH3)(3) Si). Room-temperatu re photolysis (lambda(exc) > 410 nm) in CO-saturated methanol or aceto nitrile produced (eta(1)-2,6-X(2)C(5)H(3)N)Cr(CO)(5) which subsequentl y formed Cr(CO)(6) in a secondary photochemical process (X = H or CH3) . The efficiency of pentacarbonyl formation is lower in GO-saturated c yclohexane and follows the order X = H > X = CH3. Photolysis in low-te mperature matrices resulted in an eta(6) to eta(1) pyridine ring-slipp age (lambda(exc) = 460 nm; X = H). Visible irradiation in a CO-doped m ethane matrix produced (eta(1)-C5H5N)Cr(CO)(5), while in an N-2 matrix fac-(eta(1)-C5H5N)(N-2)(2)Cr(CO)(3) is formed. Irradiation with lambd a(exc) = 308 nm produced both the ring-slippage product and also the C O-loss product (eta(6)-C5H5)Cr(CO)(2), which in a N-2 matrix is trappe d as (eta(6)-C5H5N)Cr(CO)(2)(N-2). Time-resolved infrared spectroscopy in cyclohexane revealed only the CO-loss product (lambda(exc) = 308 n m; X = H). The apparent difference in room-temperature and low-tempera ture photochemistry is explained by a rapid regeneration of (eta(6)-C5 H5N)Cr(CO)(3) from the eta(1)-intermediate. This explanation was suppo rted by laser flash photolysis experiments (lambda(exc) = 355 nm) in C O-saturated cyclohexane (Sol), where the recovery of the (eta(6)-C5H5N )Cr(CO)3 absorption follows a biphasic time profile, whereby the faste r process was assigned to the eta(1) to eta(6) transformation and the slower to the reaction of (eta(6)-C5H5N)Cr(CO)(2)(Sol) with CO. Crysta ls of (eta(6)-2,6-(CH3)(2)C5H3N)Cr(CO)(3) and (eta(6)-2,6-((CH3)(3)Si) (2)C5H3N)Cr(CO)(3) were characterized by X-ray diffraction.