LOW-TEMPERATURE ELECTRONIC-SPECTRA OF SOLID TETRAKIS(DIMETHYLDIMETHYLENEPHOSPHONIUM)DIMOLYBDENUM AND DICHROMIUM

The electronic absorption spectra of the title compounds, which contain quadruple Mo-Mo and Cr-Cr bonds, have been measured on crystalline powders pressed in KBr pellets at 5 K. The molybdenum compound has an absorption band at ca. 2000 cm-1and the low-temperature spectra reveal no less than five resolved vibrational progressions, each of which has a spacing (345 cm-1) associated with that totally symmetric vibration in the electronic excited state that is approximately describable as υMO-MO The Franck-Condon factors are of two different kinds, one associated with three of the progressions and the other with the two remaining ones. It is shown that this curious spectrum can be explained in detail using theory first discussed by Craig and Small for a transition where both orbitally allowed and vibronic contributions to intensity are of comparable magnitude. The analysis is much the same as that recently employed for the Mo2(02CCH3)4 spectrum and adds valuable support to the assignment of the electronic transition occurring at ca. 20 000 cm-1in both Mo2(02CCH3)4 and Mo2[(CH2)- P(CH3)2]4 as the 1A2u←- 1A1g(δ*←δ) transition. The spectrum of the chromium analogue is blemished by decomposition products and could not be analyzed. © 1979, American Chemical Society. All rights reserved.