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-<i>tert</i>-Butylpyridine)

The quadruply bonded dimolybdenum complexes with alkyl-substituted pyridines of the formula Mo2Cl4(R-py)4 (R-py = 4-pic (4-methylpyridine) (1), 3,5-lut (3,5-dimethylpyridine) (2), and 4-But-py (4-tert-butylpyridine) (3)) have been prepared. Nine different compounds, in which there are 11 independent molecules, have been obtained in crystalline form, and their crystal structures have been investigated by X-ray diffraction. Three types of geometric 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 bond and have a virtual symmetry D2h. The noncentrosymmetric D2d structures 1−3b have each pyridine ligand opposite to a Cl atom. The molecules 3c−f have a partially staggered D2 geometry with N−Mo−Mo−N torsion angles ranging from 10.4° to 25.2°. As a result of this work it is now clear that Mo2Cl4(R-py)4 compounds do not show a preference for D2h conformation rather than D2d conformation. In fact, they seem unusually unrestricted in their rotational conformation, and packing forces appear to have a major influence on the conformation adopted. Further clarification of this question will require spectroscopic study of solutions.