Time-resolved resonance Raman study of the δδ* excited state of Re2Cl2-8 and Re2Br2-8

Time-resolved resonance Raman (TR{sup 3}) spectra have been obtained for Re{sub 2}X{sub 8}{sup 2{minus}} (X = Cl, Br) in the A{sub 2u} ({delta}{delta}*) electronically excited state, at ambient temperature in solution. The Tr{sup 3} spectra exhibit Raman peaks that are assigned to the three symmetric vibrations of the excited state: the Re-Re stretch, Re-X stretch, and the Re-Re-X deformation. In addition, a depolarized peak attributed to an asymmetric X-Re-X bend is observed. Comparison of the TR{sub 3} results to single-crystal vibronic spectra reported by others clearly shows the effects of crystal constraints and observation time scale upon the strucutre of the {delta}{delta}* excited state. The TR{sup 3} data, obtained in solution on the nanosecond time scale, indicate that the excited state relaxes to a staggared molecular structure (D{sub 4} or D{sub 4d} symmetry). The vibronic data, obtained on single crystals under cryogenic conditions, are consistent with an eclipsed (D{sub 4h}) structure, similar to that of the ground state. A comparative TR{sup 3} study of quadruply bonded complexes, including both octahalodirhenate ions and Mo{sub 2}(PMe{sub 3}){sub 4}Cl{sub 4} (which is precluded by steric factors from significant torsional distorition about the metal-metal bond), was essential in elucidating the excited-state structures.