A study of the geometry of the [Re2Br8](2-), [Re2I8](2-) and [Mo2Cl8](4-) ions in the lowest (1)A(2u) excited state by the application of the sum-over-states method to resonance Raman band intensities

The structures of the ions [Re2Br8](2-), [Re2I8](2-) and [Mo2Cl8](4-) in the (1)A(2u) (delta delta*) excited electronic state have been investigated using a sum-over-states (S-O-S) approach. The method has been applied to the simultaneous fitting oi the (1)A(2U) <-- (1)A(1g) (delta* <-- delta) absorption band contour and the relative intensities of the n = 1-4 members of the nv(1), i.e. nv(M-M), progression in the resonance Raman (RR) spectrum produced by irradiating within the contour of this band,ln fitting the experimental data, three excited-state structural parameters, relating to the metal-metal (MM) coordinate alone, were varied: the bond length change (Delta (dimensionless) or delta) in the (1)A(2u) excited state, the wavenumber ((v) over tilde(e)) of v(1) in the (1)A(2u) excited state, and a linear non-Condon coupling parameter, m. For each of the ions, a good fit to the experimental data recorded at 77 K has been found for specified values of Delta, (v) over tilde(e) and m; in particular, the values found for delta are similar to 8 pm for [Re2Br8](2-) and [Re2I8](2-) and similar to 15 pm for [Mo2Cl8](4-). For [Re2Br8](2-) the experimental data recorded in solution at 295 K have also been fitted successfully, and values for the parameters found. No evidence has been found for a difference between the structure of this ion in the (1) delta delta* state in solution and in the solid state.