Polarized electronic absorption spectra for dimolybdenum(II) tetraacetate

Polarized spectra with rich vibrational detail are reported for two faces of single crystals of Mo2(O2CCH3)4 at temperatures down to 5 K. Evidence, which includes the orientation in space of transition moments for individual vibrational lines, hot bands, and Franck-Condon factors, is presented that the 23,000-cm-1 band is the electric-dipole-allowed 1A/sub 2u/ reverse arrow 1A/sub 1g/ or delta* reverse arrow delta with an origin at 21,700 cm-1. However, the intensities are sufficiently low that lines excited vibronically, although somewhat weaker, approach the intensity of the dipole-allowed progression. A second transition at 26,500 cm-1 is evident as well from the low-temperature spectra. The spectra of a number of dimeric carboxylate complexes of molybdenum(II) are compared and the possible cause of the exceptionally low intensity for an electric-dipole-allowed transition is discussed