Mixed chloride/amine complexes of dimolybdenum(II,II). 5. Experimental and theoretical study of the rotation conformational preferences of Mo₂Cl₄(R-py)₄ (R-py = substituted pyridine) molecules

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 D₂ (partly staggered). From the visible spectra of these solids, it is found that the D(2h) and D(2d) conformations give rise to a δ→δ* band at ~570 nm, while a D₂ conformation with a twist angle of ~20°from D(2h) has a band at ~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 D₂ 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 D₂ conformations vary greatly with solvent and temperature; (3) absorption bands at ~25 000 cm^{- 1}, 20,000 cm^-1, and 15,000-18,000 cm^-1 can be assigned to δ→d(x²- y²), π→δ*, and δ→δ* transitions, respectively.