Niobium iodide clusters

Redox Chemistry of the Group 5 Clusters Ligand Substitution Electronic and Molecular Structure Niobium Iodide Clusters NbJJ"-Materials Chemistry Derived from Soluble Metal Halide Clusters A. Higher Nuclearity Clusters Supported Cluster Materials Charge-Transfer Salt Complexes Extended Solids Chemically Modified Surfaces... 

For Nb or Ta clusters MgLig units are formed when the ligands are Cl or Br for niobium, and Cl, Br or I for the tantalum. In contrast, the niobium iodides have the MeLi4 unit, owing to the important steric effect of the iodine, as discussed later. Such MeLis units can also be prepared from valence-electron-poor transition metals such as zirconium, but in this case an interstitial element located at the center of the octahedral cluster is necessary to stabilize the cluster. ... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

Oxidized, metastable clusters are obtained for both families via solution processes near room temperature and, in parallel, these contain 14-e niobium and tantalum members, but 12-e (and 13-e) zirconium chloride clusters. Rare-earth-metal clusters don t provide such persuasive evidence small main-group Z examples are rather scarce but similar (12-14-e for R7X-12Z), while iodides centered by large metals cover a sizable range (16-20), as was rationalized earlier (see 5). 

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