PHOTOCHEMISTRY OF (ETA-1-C5CL5)MN(CO)5 AND (ETA-1-C6H5CH2)MN(CO)5 - COMPETITIVE FORMATION OF 16-INTERMEDIATES AND 17E-INTERMEDIATES

Irradiation of RMn(CO)s (R = η1-C5Cl5, η1-C6H5CH2) in alkane glasses at 95 K leads to CO loss as the only detectable photoprocess: the ring-slipped (η3-C5Cl5)Mn(CO)4and (η5-C5Cl5)Mn(CO)3or (η3-C6H5CH2)Mn(CO)4are formed, respectively. The identities of the 173 intermediates from (η1-C5Cl5)Mn(CO)5and (η1-C6H5CH2)Mn(CO)5have been established by spectroscopic means and by chemical-trapping experiments. Room-temperature irradiation of RMn(CO)5yields both CO loss and Mn-R bond cleavage, giving Mn(CO)5and R radicals. The relative importance of these two competitive primary photoprocesses is wavelength dependent. The quantum yield for Mn-R bond homolysis is largely independent of wavelength, 436-254 nm, s 0.05, whereas the CO-loss quantum yield for (η1-C5Cl5)Mn(CO)5increases from ~0.03 at 436 nm to ~0.30 at 254 nm. The relative importance of net photoproducts is also dependent on the presence of CO, which suppresses CO-loss products. From the wavelength dependence and effects of adssded CO on product distribution, it is concluded that axial or equatorial CO loss from (η1-C5Cl5)Mn(CO)5can be observed from populations of dz2: or dx2-y2orbitals, respectively, with higher energy excitation favoring equatorial CO loss. © 1990, American Chemical Society. All rights reserved.