Molybdenum(II) chloride
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Molybdenum dichloride describes chemical compounds with the empirical formula MoCl2. At least two forms are known, and both are of significance.
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[edit] 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 in that part of the transition metal series exhibit this tendency, e.g. Zr-Tc and Hf-Re.
One form of MoCl2 is the octahedral cluster Mo6Cl12. This species is polymeric, by virtue of four doubly bridging chloride ligands, but can be converted 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+.
[edit] 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-.
[edit] Related clusters
A variety of clusters are structurally related to [Mo6Cl12]2-. The tungsten analogue is known.Ta and Nb form related clusters where the halides are now edge-bridging: [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 Mo688L6, analogues of the "Chevrel phases", have been prepared by the reaction of sulfide]] sources with Mo6Cl12 in the presence of donor ligands L.[3]
[edit] References
- ^ Nannelli, P.; Block, B. P. Inorganic Syntheses 1970, XII, page170-178. ISBN 07-048517-8.
- ^ Lee, S. C. and Holm, R. H., "Nonmolecular Metal Chalcogenide/Halide Solids and Their Molecular Cluster Analogues", Angew. Chem., Int. Ed. Engl., 1990, 29, 840-856.
- ^ Saito, T., "Group 6 Metal Chalcogenide Cluster Complexes and Their Relationships to Solid State Cluster Compounds", Adv. Inorg. Chem., 1997, 44, 45-91
