The relative stabilities of rotation conformational isomers of Mo2Cl4(R-py)(4) molecules in solution have been studied. R-py represents a substituted pyridine, 4-tert-butylpyridine, 4-picoline, or 3,5-lutidine, and the MoCl2(R-py)(2) portions of each molecule have the trans configuration. In previously reported crystal structures, these molecules have been found with several different conformations: D-2h, D-2d (both eclipsed), and D-2 (partly staggered). From the visible spectra of these solids, it is found that the D-2h and D-2d conformations give rise to a delta-delta* band at similar to 570 nm, while a D-2 conformation with a twist angle of similar to 20 degrees from D-2h has a band at similar to 645 nm. For solutions in CH2Cl2 we find both bands, but the intensity ratio is strongly dependent on temperature. In addition the intensity ratio varies over a range of 100 in THF/CH2Cl2 mixtures. Calculations by the DFT method provide an estimate of the energy variation as a function of internal rotation angle from the D-2h through D-2 to D-2d conformations. The major conclusions are: (1) the D-2h conformation, although frequently found in crystals, is not stable in solution or the gas phase; (2) the relative stabilities of the D-2d and D-2 conformations vary greatly with solvent and temperature; (3) absorption bands at similar to 25 000 cm(-1), 20,000 cm(-1), and 15,000-18,000 cm(-1) can be assigned to delta-->d(x2-y2), pi-->delta*, and delta-->delta* transitions, respectively.