Compounds of Vanadium, Niobium and Tantalum

Table 22.2 Oxidation states and stereochemistries of compounds of vanadium, niobium and tantalum...

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. 

Finally, a third part of this volume offers a non-exhaustive but illushative overview of the latest hot topics of element related NMR. This includes contributions on 6,7 Li NMR, as well as metal NMR of vanadium, niobium and tantalum compounds. 

Sheng and Zajacek [182] have shown that vanadium complexes are somewhat superior catalysts to molybdenum. Both give good selectivity to amine oxide but reaction rates are faster when vanadium complexes are used. Compounds of tungsten, niobium and tantalum were poorer catalysts whereas chromium, cobalt, manganese and iron complexes were ineffective. 

Vanadium, niobium, and tantalum are metals which possess the capacity of forming both basic and acidic oxides, and, as they also display several degrees of combining power, the types of compounds they produce are very varied and often complex. The chemistry of these elements is, therefore, of considerable interest. In consequence of the difficulty experienced in separating niobium and tantalum satisfactorily, the compounds of these two elements have not been extensively explored their further investigation certainly offers an attractive field for research. 

Vanadium, niobium and tantalum - All these elements have magnetic nuclei occurring at virtually 100% natural abundance, although the high quadrupole moment of tantalum has tended to restrict investigations to those of compounds with highly symmetric coordination around the metal. 

In this article the author has been using the traditional ylide-ylene nomenclature as well as the modern phosphorane-arsorane-stiborane formalism. No attempt has been made to discriminate between one or the other of these modalities because it is felt that both offer a sufficiently clear description of at least those species that are derived from the main Group Vb elements. (Fortunately no Nli,s or BiR5 had to be given names.). However, when this procedure was followed with the vanadium, niobium, and tantalum compounds, an interesting problem arose because the authors... 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

Niobium and tantalum provide no counterpart to the cationic chemistry of vanadium in the -t-3 and -t-2 oxidation states. Instead, they form a series of cluster compounds based... 

As is usual with transitional elements, vanadium possesses considerable freedom in its valencies with niobium and tantalum, however, this freedom is less marked. This can be seen in the variation of the readiness displayed by the compounds of these elements to undergo reduction ... 

Vanadium oxyfluorides also react with hydrogen peroxide to yield complex compounds.2 These are not so well defined as in the case of the peroxyfluorides of niobium and tantalum. 

Although there are few five-membered metallacycles reported for group 5, there are some examples of metallacyclopentane compounds for niobium and tantalum. The reaction of the labile vanadium naphthalene complex [CpV(Ci0H8)] with ethene provided the unusual binuclear complex [Cp2V2(p-C4H8)2] (34 [Eq. (13)], Fig. 9) in which the two vanadium centres are bridged by two butanediyl groups.38... 

Considering the presently available knowledge on vanadium(V) compounds, reaction (58b) seems to be more attractive, as compounds of types (RO)3VO, (RO)3V=NR, etc., are well documented for both R = alkyl and R = silyl (4, 81), whereas only very few VX6 species are known. The pertinent chemistry of the congeners niobium and tantalum offers examples for pentacoordination as well as for ylide formation. 

This procedure has been used to prepare many other chlorides, e.g., tungsten hexachloride, vanadium tetrachloride, and the pentachlorides of niobium and tantalum. However, it is normally much easier to prepare chlorides than fluorides. This method will therefore find application only where this generalization is not applicable. In particular, it will find application where the chloride is thermally unstable under normal chlorination conditions and must be prepared at or below room temperature. This method has been used to advantage for the preparation of ReCle and the new compound osmium pentachloride. ... 

Oxidative addition reactions for group VA compounds within the context of the discussion found in 5.8.2.9.1 are presented in 10.3.3.1 and 10.3.3.2, including some cases of reactions that may not be true oxidative additions. The discussion on vanadium is separated from its heavier cousins, niobium and tantalum, since the chemistry of V is distinct from that of Nb and Ta. Oxidative additions across metal-metal multiple bonds and involving clusters are not covered. 

Of the halogens, only the strongly oxidizing fluorine produces a pentahalide of vanadium, and the other vanadium(V) compounds are based on the oxohalides and the pentoxide. The pentoxide also gives rise to the complicated but characteristic aqueous chemistry of the polymerized vanadates (isopolyvanadates) which anticipates the even more extensive chemistry of the poly molybdates and poly tungstates this is only incompletely mirrored by niobium and tantalum. 

---