Photochemistry of (η¹-C₅Cl₅)Mn(CO)₅and (η¹-C₆H₅CH₂)Mn(CO)₅: Competitive Formation of 16- 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 d_z²: or d_x2-y2orbitals, respectively, with higher energy excitation favoring equatorial CO loss.