Molybdenum(II) chloride

Molybdenum dichloride describes chemical compounds with the empirical formula MoCl₂. At least two forms are known, and both have attracted much attention from academic researchers because of the unexpected structures seen for these compounds and the fact that they give rise to hundreds of derivatives. In general, compounds composed only of an element and chloride are considered valuable.

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Structure

At first glance, a species with the formula MoCl₂ might be expected to adopt a close-packed structure, e.g. the cadmium chloride, rutile, and cadmium iodide motifs. Molybdenum(II), which is a rather large ion, prefers to form compounds with metal-metal bonds, i.e. metal clusters. In fact all "lower halides" (i.e. where halide/M ratio is <4) in the "early transition metal series (Ti, V, Cr, Mn triads).

One form of MoCl₂ has the formula Mo₆Cl₁₂. This species is polymeric consisting of cubic Mo₆Cl₈⁴⁺ clusters interconnected by chloride ligands that bridge from one cluster to cluster. This material converts readily to salts of the dianion [Mo₆Cl₁₄]^2-. In this anion, each Mo bears one terminal chloride but is otherwise part of an Mo₆ octahedron embedded inside a cube defined by eight chloride centers. Thus, the coordination environment of each Mo is four triply bridging chloride ligands, four Mo neighbors, and one terminal Cl. The cluster has a "magic" electron count of 24e, four being provided by each Mo²⁺.

Synthesis and reactions

Mo₆Cl₁₂ is prepared by the reaction of molybdenum(V) chloride with molybdenum metal:

12 MoCl₅ + 18 Mo → 5 Mo₆Cl₁₂

This reaction proceeds via the intermediacy of MoCl₃ and MoCl₄, which also are reduced by the presence of excess Mo metal. The reaction is conducted in a tube furnace at 600 - 650 °C.^[1]

Once isolated, Mo₆Cl₁₂ undergoes many reactions with retention of the Mo₆]¹²⁺ core. Heating in concentrated HCl gives (H₃O)₂[Mo₆Cl₁₄]. The terminl chloride ligands, labeled "ausser" are readily exchanged:

(H₃O)₂[Mo₆Cl₁₄] + 6 HI → (H₃O)₂[Mo₆Cl₈I₆] + 6 HCl

Under more forcing conditions, all 14 halides can be exchanged to give salts of [Mo₆Br₁₄]^2- and [Mo₆I₁₄]^2-.

Related clusters

A variety of clusters are structurally related to [Mo₆Cl₁₂]^2-. The tungsten analogue is known. Ta and Nb form related clusters where halides are bridge edges of the Ta₆ octahedron vs faces. The resulting formula is [Ta₆Cl₁₈]^4-.

Sulfido and selenido derivatives are also well studied. [Re₆Se₈Cl₆]^4- has the same number of valence electrons as does [Mo₆Cl₁₂]^2-.^[2]

The Mo-S clusters Mo₆S₈L₆, analogues of the "Chevrel phases", have been prepared by the reaction of sulfide sources with Mo₆Cl₁₂ in the presence of donor ligands L.^[3]

References

1. ^ Nannelli, P.; Block, B. P. Inorganic Syntheses 1970, XII, page170-178. ISBN 0-07-048517-8.
2. ^ Lee, S. C. and Holm, R. H., "Nonmolecular Metal Chalcogenide/Halide Solids and Their Molecular Cluster Analogues", Angewandte Chemie, International Edition in English, 1990, volume 29, pages 840-856.
3. ^ Saito, T., "Group 6 Metal Chalcogenide Cluster Complexes and Their Relationships to Solid State Cluster Compounds", Advances in Inorganic Chemistry, 1997, volume 44, 45-91