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) do.
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 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+.
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Read more about this topic: Molybdenum(II) Chloride
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