Photochemistry of Metal-Metal Bonded Complexes. 5. Cleavage of the M-M Bond in (OC)₅M-M(CO)₃L by Irradiation into a Low-Lying M → L Charge-Transfer Band

The photochemistry and some electronic spectral data are reported for the complexes Mn₂(CO)₈(phen) (phen = 1,10-phenanthroline), Re₂(CO)₈(phen), and Re₂(CO)₈(biquin) (biquin = 2,2′-biquinoline). All three complexes exhibit very intense, solvent sensitive, visible absorption maxima. For example, Mn₂(CO)₈(phen) has a maximum at 18.87 kK in acetone which shifts to 16.72 kK in isooctane with a molar absorptivity of about 12 000 1 mol^-1 cm^-1. The lowest energy absorption is associated with a M-Mσ_b→π^*L transition. Emission from either the pure solid or 77 K EPA solutions of Re₂(CO)₈(phen) is found and is associated with radiative decay from the lowest CT state. Consistent with the depopulation of a σ_b orbital with respect to the M-M bond we find that the homolytic M-M bond cleavage obtains upon electronic excitation of the complexes in solution at 298 K. The quantum efficiency for disappearance of the M-M bonded species is independent of the excitation wavelength in the range 313 to 633 nm. The quantum yields for disappearance in degassed CH₂Cl₂/CCl₄ (1/2 by volume) are 0.96 ± 0.10, 0.17 ± 0.02, and 0.028 ± 0.004 for Mn₂(CO)₈(phen), Re₂(CO)₈(phen) and Re₂(CO)₈(biquin), respectively. Under these conditions the only metal-containing photoproducts are M(CO)₅Cl and ClM(CO)₃L Flash photolysis of M₂(CO)₈(phen) in degassed benzene solution gives M₂(CO)₁₀ in good yield.