Tungsten metal-atom cluster compounds

There are a number of compounds of low-valent (mainly oxidation state II) molybdenum and tungsten, which contain metal-atom clusters. The key structural unit is that shown in Fig. 26-C-13, consisting of an octahedron of metal atoms with a bridging atom on each triangular face the entire unit has full Oh symmetry. Detailed dimensions are available for the (Mo6Cl8)4+ unit in molybdenum dichloride64 and for the (Mo6Br8)4+ unit in... 

In an ever increasing number of compounds tolylidyne ligands triply-bridge a triangle of metal atoms, one of which is tungsten and the other two may or may not involve the same element. Thus two types of core structure are possible fM2W(y3-CR)l (Class A) or [M1M2W(y3-CR)] (Class B). Two methods are available for the synthesis of clusters of Class A. viz... 

Clearly, U is the biggest number in the cycle and is the main driving force for the formation of ionic compounds. Nevertheless, the other factors can tip the balance one way or another. For example, AHSub is particularly large for the transition metals niobium, tantalum, molybdenum, tungsten, and rhenium, with the result that, in their lower oxidation states, they do not form simple ionic compounds such as ReCl3 but rather form compounds that contain clusters of bonded metal atoms (in this example, Re3 clusters are involved, so the formula is better written ResClg). 

Table IV lists specific examples of compounds related through this form of dimensional reduction, By far, the majority of these are zirconium chloride and iodide phases, in which case lower main group and even transition metals have been found to incorporate as interstitial atoms. A few analogues are known with hafnium (135), and very recently it has been shown that nitrogen can be substituted for carbon in tungsten chloride clusters adopting the centered trigonal-prismatic geometry (see Fig. 2) (32). It is hoped that a variability similar to that exposed for the octahedral zirconium clusters will be attainable for such trigonal-prismatic cluster phases.

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