Pressure tuning spectroscopy of the metal-metal quadruple bond

Abstract

The results obtained from the application of high pressure in optical absorption studies of compounds with metal-metal quadruple bonds are reviewed. The different degrees of balance between intermolecular and intramolecular effects of compression in different cases are highlighted. A consistent effect observed for [Re2Cl8]2−, [Mo2Cl8]4− , and related compounds is a continuous shift to Iower energy (red) in the δ to δ* excitation with increasing pressure. This observation is interpreted in terms of significant van der Waals interactions between the molecular chromophore and its surroundings, leading to relative stabilization of the more polarizable excited state. Changes in the intensity of the δ to δ* peak are not correlated with the changes in position. The intensity changes become consistent only at relatively high pressures, and this falloff is attributed largely to compression and deformation of the cell gasket. In certain cases, such as [Re2I8]2− and Re2(piv)4X2, piv = pivalate, X= CI, Br, rearrangement to a new species is observed upon the application of high pressure. These rearrangements are all reversible upon pressure release, but they involve different degrees of interaction between adjacent molecules.