Pressure tuning spectroscopy of binuclear metal cluster compounds

The effect of pressure has been measured on the absorption spectra of (n-Bu4N)2Re2X8 (X = Cl, Br, I), K4Mo2Cl8, (NH4)4Mo2Br8, and Re2[CCO2 (CH3)3]2X8(X' = Cl, Br). For the rhenium and molybdenum halides in the solid state a distinct shift to lower energy (red shift) was observed for the δ→δ* excitation. In solution the shift of the δ→δ* was red in CH2Cl2 but exhibited a complex behavior in the associated solvents CH3CN and CH3NO2 which we attributed to solvation of the ion. The dipivalate molecules in CH3CN exhibited a red shift as one might expect as they are less solvated. The π→π* excitations in the rhenium bromide and iodide showed a blue shift which increased with pressure. We discuss the observed shifts in terms of the relative importance of intra and intermolecular interactions