Magneto-Optical Investigation of the Exchange-Coupled Dimer Cs(3)Mo(2)Br(9).

The electronic spectrum of the confacial bioctahedral complex Cs(3)Mo(2)Br(9) has been investigated by single-crystal absorption, Zeeman, and MCD spectroscopies. A total of seven distinct band regions were resolved and assigned to transitions essentially within the t(2)(2)t(2)(2) configuration that arises when the pair of t(2)(z)() orbitals in the t(2)(3)t(2)(3) configuration is decoupled by strong Mo-Mo sigma bonding. The excited state separations, and in particular the orbital g values, are sensitive to Mo-Mo pi bonding and show clearly that the pi bonding is weaker in the bromide than in the chloride complex.