Molybdenum 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... 

Despite its spectacular present, it should be remembered that metal atom cluster chemistry also has a very early origin. The earliest point to which I have been able to trace it is the period 1857 1861 when a Swedish chemist, Christian Bloomstrand, discovered the dichloride and dibromide of molybdenum. Based on the significant observation that these contained two kinds of halogen, one precipitable by silver ion and the other not, in a 1 2 ratio, he concluded that the minimum acceptable molecular formula for these compounds was M03X6, and that this consisted of a... 

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). 

As used here cluster refers to molecules that contain at least 3 metal atoms and electron configurations suitable for the formation of metal-metal bonds. Molybdenum readily forms strong homoleptic metal-metal bonds (see Homoleptic Compound) and so it is not surprising that molybdenum is readily incorporated into cluster stractures (see Polynuclear Organometallic Cluster Complexes). 

In the high-valence cluster species, metal atoms forming the metal network appear to have positive intermediate oxidation states. However, metal oxidation states in cluster are always lower than those characteristic for the same metals in classical mononuclear complexes. The ligands associated to this class of cluster are normally good cr-donors that according to the classification of Pearson would have intermediate hardness. Among these compounds the most frequent are the halides, specially chlorides and bromides. An example of this cluster class is the molybdenum species [Mo6Xs] shown in Fig. 2.1c. 

The situation is very similar in the Chevrel phases. These are ternary molybdenum chalcogenides A,.[Mo6Xg] (A = metal, X = S, Se) that have attracted much attention because of their physical properties, especially as superconductors. The parent compound is PbMo6Sg it contains Mo6Sg clusters that are linked with each other in such a way that the free coordination sites of one cluster are occupied by sulfur atoms of adjacent clusters (Fig. 13.9). The electric properties of Chevrel phases depend on the number of valence electrons. With 24 electrons per cluster (one electron pair for each edge of the... 

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