Photochemistry of Tetracarbony Ibis (cyclopentadienyl) dichromium (I) and Related Compounds: Labilization of Carbon Monoxide

Photochemistry of three related compounds, (η-C₅H₅)2Cr₂(CO)4, (η-C5Me5)2Cr₂(CO)4, and (η-C₅H₅)(η-C5Me5)Cr₂CO)4, having a chromium-chromium triple bond has been examined, and the principal result is that the CO ligands are labilized by photoexcitation. Spectroscopically, the complexes are similar and exhibit a visible absorption at ~600 nm (e = 300 M-1 cm-1) and a near-ultraviolet absorption at ~400 nm (ε ≈ 104 M-1 cm-1) in alkane solution. Homolytic scission of the metal-metal triple bond is ruled out as a primary photoprocess by the facts that (i) irradiation of (η-C₅H₅)(7)-C5Me5)Cr₂(CO)4 in alkane solution does not yield (r?-C3H5)2Cr₂(CO)4 and (η-C5Me5)2Cr₂(CO)4 and (ii) irradiation of both (η-C₅H₅)2Cr₂(CO)4 and (i?-C5Me5)2Cr₂(CO)4 as a 1/1 mixture in alkane solution does not yield (η-C5H3)(η-C5Me5)Cr₂(CO)4. Labilization of CO upon photoexcitation is established by irradiation of (η-C₅H₅)2Cr₂(CO)4 or (C5Me5)2Cr₂(CO)4 in alkane solution under a 13CO (90% 13C, 10% 12C) atmosphere. Quantum yields for CO exchange are generally low and dependent on the excitation wavelength. The quantum yields for CO exchange for (η-C5Me5)2Cr₂(CO)4 are <2 X 10-4 at 633 nm, (1.2 ± 0.3) X 10-3 at 458 nm, and (4.0 ± 1.0) X 10-2 at 366 nm. The wavelength dependence is consistent with dissociative loss of CO from an upper excited state. When the complexes are irradiated in degassed alkane solution (in the absence of CO), decomposition occurs with an initial quantum yield similar to that for CO exchange. The presence of CO generally suppresses decomposition but not completely. The (η-C5Me5)2Cr₂(CO)4 is the least susceptible to decomposition in the presence of CO.