Group 5 metal halide clusters oxidation states

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. 

Those in which the metal atoms are in somewhat higher oxidation states (+2 to +4) and the ligands are typically halide, sulfide, or oxide ions and some others of the same ilk as those in mononuclear Werner complexes. Clusters of this type are most common among the early transition elements, groups 5-7. 

There are two main classes of cluster compounds — those where the metal atoms have a low formal oxidation state, in which case the ligands are almost always CO groups and those with metals that have a high oxidation state, ranging from +2 to +4. In this case, the ligands are usually halides, sulfides, oxide ions, or ligands from a monomolecular Werner complex (see Chapter 9 for details on a Werner complex). 

Most transition metal clusters not containing carbonyl groups are metal halide derivatives in which the transition metal has a relatively low oxidation state. These are prepared by reduction of higher metal halides or other derivatives with the metal in a relatively normal oxidation state, for example,... 

Clusters of the group V-VII metals in high formal oxidation states, stabilised by 7t-donor ligands such as oxide, sulphide, or halides. Examples here include [Nb6Cli2] +, [WeBrgJ +and [Re3Cl9] +. 

We have found that the main group metal and metalloid reductants mocury, bismuth, and antimony are highly effective " in reducing WCIe or M0CI5 at surprisingly lower temperatures than commonly used in the solid-state synthesis of early transition metal cluster halides. BorosUicate ampules can be substituted for the more expensive and less easily sealed quartz ampules at these lower temperatures, and the metals and metalloids are not as impacted by oxide coatings that inhibit sohd-state reactions with more active metals. These lower temperatures may allow access to kinetic products, such as trinuclear clusters, instead of thermodynamic products. 

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