Niobium complexes oxidation states

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

In a third complex reaction, diphenylacetylene and acetonitrile have been observed to condense into the ty6-HC(QHs) C(QH5))3C(NH)(CH3) ligand, bonded to niobium (Fig. 13)2255. Among a number of resonance hybrids can be included the simplified form XIX, which contains a pentadienyl-metal interaction. Of course, the ligand could also be considered to be isoelectronic with hexatriene or the hexatriene dianion, depending on the metal oxidation state (cf., Fig. 6). A great deal of buckling is present in the con-... 

In general, Group 5 fluorides tend to adopt a preference for higher oxidation states, and as a consequence, over the past few years, there have been virtually no reported examples of low valent fluoro complexes of vanadium, niobium or tantalum. 

No derivative in which niobium or tantalum displays an oxidation state of —II has been reported so far, while carbonyl complexes derived from [M(CO)s] represent the only coordination compounds for these metals in oxidation state —III. 

Element 105, (eka-tantalum) should resemble tantalum and niobium, with the pentavalent state being the most important. It should exhibit several oxidation states, such as +4 and +3, in addition to the more stable +5 state. There should be an extensive range of complex ions, offering a rich chemistry. 

Niobium occurs in oxidation state (V) in the dissolver solution and does not extract into pure TBP/OK phases to a significant extent. Values of ca. 5x10 have been quoted for Dn6 between 20% TBP/OK and 2 M HNO3. Inextractable polymeric hydroxy complexes of Nb are formed but the presence of silicates or HDBP can lead to some extraction of niobium species. A model for the extraction by HDBP is provided by HELP, which was thought to extract Nb according to equation (159) on the basis of IR studies. HDBP might similarly lead to some synergic extraction of Nb " in TBP/OK but, as mentioned above, free HDBP will not be available in the first process cycle,and in practice problems do not usually arise with niobium. 

In the case of manganese(I), with an electronic configuration of 3d , the four-electron donor group RCO— would meet the EAN rule. Indirect support to the suggestion of a small steady state concentration of this type of intermediate along the reaction profile comes from isolation of stable rj -coordinated acyl complexes of ruthenium(II) , of group IVA transition elements and of thorium , all in the highest oxidation state. Also > -acyl complexes of vanadium(I) ° and niobium(V) have been reported. Most of the molybdenum acyl complexes are of the type . 

A more recent example involving a-elimination is shown in equation 10.12. Here, Ag(I) is used to induce alkylidene formation by first oxidizing Ti to a (f1 state, which is followed by a-elimination and then RE of alkane. Similar chemistry occurs with vanadium and niobium complexes.31... 

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