Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 4. Rotational Isomers of Mo(2)Cl(4)(R-py)(4) (R-py =...

The quadruply bonded dimolybdenum complexes with alkyl-substituted pyridines of the formula Mo(2)Cl(4)(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)) 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 D(2)(h)(). The noncentrosymmetric D(2)(d)() structures 1-3b have each pyridine ligand opposite to a Cl atom. The molecules 3c-f have 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 Mo(2)Cl(4)(R-py)(4) compounds do not show a preference for D(2)(h)() conformation rather than D(2)(d)() 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.