Mixed chloride/amine complexes of dimolybdenum(II,II). 4. Rotational isomers of Mo2Cl4(R-py)(4) (R-py=4-picoline, 3,5-lutidine, and 4-tert-butylpyridine)

The quadruply bonded dimolybdenum complexes with alkyl-substituted pyridine s of the formula Mo2Cl4(R-py)(4) (R-py = 4-pic (4-methylpyridine) (1), 3,5- lut (3,5-dimethylpyridine) (2), and 4-Bu-t-py (4-tert-butylpyridine) (3)) h ave been prepared. Nine different compounds, in which there are 11 independ ent molecules, have been obtained in crystalline form, and their crystal st ructures have been investigated by X-ray diffraction, Three types of geomet ric isomers which differ by the angle of internal rotation about the Mo-Mo axis have been recognized in these structures. The eclipsed structures 1-3a display pyridine ligands opposite to each other across the metal-metal bon d and have a virtual symmetry D-2h The noncentrosymmetric D-2d structures 1 -3b have each pyridine ligand opposite to a Cl atom. The molecules 3c-f hav e a partially staggered D-2 geometry with N-Mo-Mo-N torsion angles ranging from 10.4 degrees to 25.2 degrees. As a result of this work it is now clear that Mo2Cl4(R-py)(4) compounds do not show a preference for D-2h conformat ion rather than D-2d conformation. In fact, they seem unusually unrestricte d in their rotational conformation, and packing forces appear to have a maj or influence on the conformation adopted. Further clarification of this que stion will require spectroscopic study of solutions.