Chemical and X-ray crystallographic studies of molybdenum(II) halides, which are based on the well-known (Mo₆Cl₈)⁴⁺ cluster, have been carried out.

Contrary to previous reports, the reactions of 2,2'-bipyridyl with the halides (Mo₆Cl₈)Cl₄ and (Mo₆Cl₈)I₄ yield, even under mild conditions, bipyridylium salts of chloromolybdic(II) and iodomolybdic(II) acids respectively: (BipyH)₂((Mo₆Cl₈)X₆) where X = Cl, I and Bipy = 2,2'-bipyridyl. The reactions are complicated by the formation of mixtures of products, which are mainly various crystalline forms of the bipyridylium salts. An amorphous product may be a true mono-bipyridyl complex.

An unusual oxidation occurs during the reactions of triphenylphosphine (Ph₃P) and triphenylarsine (Ph₃As) with (Mo₆Cl₈)Cl₄ and (Mo₆Cl₈)I₄. Infra-red spectral and X-ray powder photographic studies show that the oxidized ligand complexes, (Mo₆Cl₈)X₄(Ph₃Z0)₂ (X = Cl, I; Z = As,P), are formed except under conditions in which both molecular and chemically-bound oxygen is rigorously excluded.

The conditions required to coordinate more than two neutral unidentate ligands to the (Mo₆Cl₈)⁴⁺ cluster have been examined. It proved possible to obtain new ionic complexes under a range of conditions. The six-fold coordination of the (Mo₆Cl₈)⁴⁺ cluster is maintained in these compounds (e.g. ((Mo₆Cl₈)I₃(triphenylphosphine oxide)₂(pyridine))⁺I⁻) by the ionization of one or more of the terminal halogen atoms in the molybdenum(II) halide starting material (e.g.(Mo₆Cl₈)I₄).

The X-ray single crystal structures of two isomorphous salts, (BipyH)₂(( (Mo₆Cl₈)X₆) (X = Cl,I), have been solved using the difference Patterson method. To solve another crystalline modification of the chloro-salt, the (Mo₆Cl₈) cluster was constrained to its established geometry with its centroid fixed at the origin of the unit cell. This rigid group of atoms was then rotated by the least-squares refinement of the three orientation-defining angles.

The three structures contain discrete ((Mo₆Cl₈)X₆)²⁻(X = Cl,I) and (C₁₀H₉N₂)⁺ (bipyridylium) ions. The anions consist of highly-symmetric (Mo₆Cl₈) clusters (Mo-Mo = 2.606, Mo-Cl = 2.48 Ao), with six terminal halogen atoms (X) bound by single covalent bonds to the molybdenum atoms (Mo-Cl = 2.423, Mo-I = 2.737 Ao). The bipyridylium cations are twisted from perfect cis conformations in all three structures. The average dihedral angle between the two rings is 13 degrees. Further details of the geometries of the anion and cation are discussed.

The ionic packing in the three crystals is dominated by the bulky anions. These are arranged in expanded "hexagonal close-packed" layers with the cations centred on approximately trigonal holes in this array. The two crystalline modifications of the chloro-salt differ in the orientation of the bipyridylium cations in these layers.