Spectroscopy and structure of quadruply bonded complexes under extreme pressure (Re sub 2 X sub 8 sup 2 minus , Mo sub 2 Cl sub 4 (PMe sub 3 ) sub 4 )

Application of pressure (up to 150 kbar, where 1 kbar = 986.9 atm = 0.1 GPa) to solutions of Re{sub 2}X{sub 8}{sup 2{minus}} (X = F, Cl, Br) and Mo{sub 2}Cl{sub 4}(PMe{sub 3}){sub 4} results in smooth, continuous changes in experimental observables related to metal-metal bond length and, for the Re complexes, torsional angle (X-Re-Re-X dihedral angle). Resonance Raman studies show that increasing pressure engenders an increase in the frequency of the metal-metal stretching mode for all of the species studied, and this change is empirically correlated with a decrease in metal-metal bond length. Large red shifts in the energy of the {sup 1}({delta} {yields} {delta}*) transitions with increased pressure for Re{sub 2}Br{sub 8}{sup 2{minus}} and Re{sub 2}Cl{sub 8}{sup 2{minus}}, in contrast to the relative insensitivity of Re{sub 2}F{sub 8}{sup 2{minus}} absorption and Mo{sub 2}Cl{sub 4}(PMe{sub 3}){sub 4} fluorescence transitions, imply a tendency for the former complexes to distort torsionally from an eclipsed configuration toward a staggered configuration due to pressure-induced steric repulsion of the chloride and bromide ligands. Integrated absorption intensities further show that, at higher pressures, even Re{sub 2}F{sub 8}{sup 2{minus}} undergoes some torsional distortion, whereas the steric repulsion of adjacent phosphines located at a 90{degree} dihedral angle on opposite Mo atoms prevents Mo{sub 2}Cl{sub 4}(PMe{sub 3}){sub 4} from distorting along the torsional coordinate. 28 refs., 7 figs., 1 tab.