Oxidation states 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. 

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

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... 

Indicate the position of vanadium, niobium, and tantalum in Mendeleev s periodic table of the elements, the electron configurations and size of their atoms, and their oxidation states. 

Compared to vanadium, the higher oxidation states are much more frequently and the lower ones much less frequently encountered in niobium and tantalum. Niobium and tantalum are also much more prone to extended metal-metal bonding in their lower oxidation states V6-based clusters, for example, remain undiscovered. Little resemblance is found to the group VB elements phosphorus and arsenic. 

The chemistry of niobium and tantalum ranges from oxidation state +V to —III, but with no species of oxidation state —II presently known (Table 1). The largest number of molecular compounds, by far, is found for oxidation state V. The very reactive pentahalides provide the most convenient entry to the molecular chemistry of these metals. 

Niobium and tantalum halides form adducts with various nitrogen donor ligands including aliphatic and aromatic amines nitriles, Schiffs bases and imidazoles (Table 5). The reactions of MXS with pyridine and related ligands such as bipy or phen depend critically on the reaction conditions. With py at low temperature MX5 (X = Cl, Br) yielded 1 1 adducts that are rapidly reduced to [MX4(py)2] on increasing the temperature, with formation of l-(4-pyridyl)pyridinium halide. Similarly, bipy and phen reduced the metal in MeCN to oxidation state +IV and formed monoadducts of type [MX bipy)] at room temperature, while at 0°C the same reactions yielded [NbCls(bipy)(MeCN)] and [TaX5(bipy)(MeCN)J (X = C1 or Br). NbBrs and Tals formed [MX5(bipy)2], which were formulated as the eight-coordinate [MX4(bipy)2]X.1 Reduction of the metal can however be prevented, even at room temperature,... 

The chemistry of niobium and tantalum in their lower oxidation states is expanding rapidly. The first structurally characterized molecular Nb111 derivative was reported in 1970,525 while Nb111 and Tam halide adducts were described in 1973580 and 1978, respectively.581... 

Table 43 Various Niobium and Tantalum Clusters with Non-integral Oxidation States...

The number of authenticated niobium and tantalum coordination compounds in oxidation state +1 is very limited. They were generally obtained through reduction of [MCl4(dmpe)2], [TaCl2(dmpe)2] or Nb(OC6H3Me2-3,5)2(dmpe)2.731a... 

Compounds containing niobium or tantalum in negative formal oxidation states -I and -III are mainly metal carbonyl anions. Although these are organometallic derivatives, the report of efficient procedures for the synthesis of [M(CO)6] since the review of Labinger8 merits mention, as it can be anticipated that these highly reduced and reactive species will be important precursors of a large variety of new coordination compounds and metal clusters. 

The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717 The coordination polyhedron is octahedral (Nb—C 2.098(5) A CNbC 89.2(2)°). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v(CO) is found in the IR (1854 and 1852 cm-1 for Nb and Ta respectively). Comparable spectra are observed for Na[M(CO)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(CO)6] appears to be the most labile carbonyl of the group VA analogs. 

Note Since the chapter was completed, relevant papers concerning the chemistry of niobium and tantalum in various oxidation states with bulky anionic oxygen donors [2,6-dialkylaryloxo731 or silox (Bu SiO)732] have appeared. Reviews concerning the analysis and classification of X-ray data of niobium733 and tantalum734 are also now available. 

The predominant oxidation stale of the element is (V). There is some evidence that the (IV) state is obtained under certain reduction conditions. When the pentapositive form is not in the form of a complex ion it may exist in solution as PaC>2+. The compounds are very readily hydrolyzed in aqueous solution yielding aggregates of colloidal dimensions, thus showing marked similarity to niobium and tantalum in this respect. These properties play a dominant role in the chemical properties of aqueous solution, because the element is so easily removed from solution by hydrolysis and adsorption Protactinium coprecipilates with a wide variety of substances, and it seems likely that the explanation for this lies in the hydrolytic and adsorptive behavior. 

The atoms of the vanadium group metals have five valence electrons. In vanadium (Z — 23) and niobium (columbium, Z = 41), these valence electrons lie beyond ra re-gas cores, whereas in tantalum (Z = 73), they lie beyond the xenon core which has been augmented by fourteen 4/ electrons. The +5 oxidation state is characteristic of this family for niobium and tantalum it is the only oxidation state of importance. Oxidation is often regarded as removal of five valence electrons, followed by coordination of the pentapositive ion (which cannot exist for appreciable time in chemical systems) to basic groups which are present (H2O, OH, Cl, etc.). Although such a description almost certainly has very little resemblance to the actual path of oxidation of these metals, it is clerically convenient and may be used if not taken literally. In the same way, the lower oxidation states of vanadium may be considered vanadium atoms with the two 4s electrons removed, and with additional removal of one or two 3d electrons. 

Table 18-B-l Oxidation States and Stereochemistries of Niobium and Tantalum...

Niobium and tantalum halides also fonn adducts with numerous N-donors. Their reactions with pyridine and related ligands (bipyridine, phenanthroline, 7-azaindole ) depend critically on the reaction conditions. Indeed, aromatic amines have a tendency to reduce the metal to oxidation state IV especially for niobium but the reduction can be prevented, even at rt, by an appropriate choice of the solvent (equations 2a-c). Imide adducts M(NR)Cl3L2 are obtained with primary or secondary amines. ... 

The development of the chemistry of niobium and tantalum in their lower oxidation states, three or less, has long been precluded by the lack of convenient starting materials. Indeed, stoichiometric trihalides MX3 (X = Cl, Br) are only incidental compositions. They are obtained by reduction of the pentahalides or disproportionation of the tetrahalides, and are polymeric and rather inert. 

The number of niobium and tantalum coordination compounds in oxidation states lower than II is limited, and they often are organometallic derivatives involving carbonyl, cyclopentadienyl, or alkenic ligands. 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

The results of solid state reactions of protactinium dioxide and pentoxide with other metal oxides (89, 93-96) support the view that the oxide systems of protactinium resemble those of other actinide elements rather than those of niobium and tantalum. However, when assessing results of this type one must always bear in mind the relative ionic radii of the respective M " and M + ions since they obviously play a large part in determining the structures of the complex phases. This comment applies equally well, of course, to the structural properties of other types of compound and in particular to the high coordination numbers exhibited by protactinium(V) in its chloro and nitrato complexes. 

The coordination chemistry of the intermediate oxidation states of niobium and tantalum is an unexplored field with respect to bipyridyl and phenanthroline. 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

The lower oxidation states of niobium and tantalum are unimportant compared to the -1-5 state. Because of the general insolubility of the oxides and (he lack of stable lower oxidation slates, (here is little solution redox chemistry. Niobium(III) does appear to form upon the reduction of niobium(V) with zinc, and is stable in the cold in the absence of air, but if the solution is heated, decomposition occurs with precipita-lion of mixed oxides. 

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