Halides of niobium

The lower halides of niobium and tantalum consist of tightly bound clusters of metal atoms, with metal-metal distances close to those found in the metal. They contain ions with average oxidation numbers between +III and +1 (Table 43). Their size depends on the valence electron concentrations (VEC) that are available on the metal atoms for M—M bonding, and on the halide-metal ratio.644 Several reviews have been devoted to the clusters of early transition metals.3,643... 

The numerous alkoxide halides of niobium and tantalum containing the atoms of all the known halogenes from F to I and a broad range of radicals are considered in the Table 12.17. 

Buslaev, Y.A., Kopanev, VD. and Tarasov, VP. (1971) 93Nb Nuclear magnetic resonances in solutions of the halides of niobium. Chem. Commun., 1175-1177. 

Other Halides of Niobium(v) and Tantalum(v). All six of these are yellow to brown or purple-red solids best prepared by direct reaction of the metals with excess of the halogen. The halides melt and boil at 200-300° and are soluble in various organic liquids such as ethers, CC14, etc. They are quickly hydrolyzed by water to the hydrous pentoxides and the hydrohalic acid. The chlorides give clear solutions in concentrated hydrochloric acid, forming oxo chloro complexes. 

F. Fairbrother, Halides of Niobium and Tantalum, in Halogen Chemistry , ed. V. Gutmann, Academic, London, 1967, p. 123. 

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. 

Hydrofluoride synthesis is based on the simultaneous fluorination by ammonium hydrofluoride of niobium or tantalum oxides with other metals compounds (oxides, halides, carbonates etc.) [105]. Table 13 presents some properties of ammonium hydrofluoride, NH4HF2 [51, 71]. Ammonium hydrofluoride is similar to anhydrous HF in its reactivity, but possesses some indisputable advantages. The cost of ammonium hydrofluoride is relatively low, it can be dried and handled easily, recycled from gaseous components, and its processing requires no special equipment. 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

Fig. 6.1b) in which twelve inner ligands bridge the edges of the Me octahedron, and six outer ligands occupy apical positions, predominate. These units are found in reduced zirconium, niobium, tantalum, and rare-earth halides, and niobium, tantalum, molybdenum and tungsten oxides [la, 6, 10]. 

The octahedral metal clusters that have long been familiar features of the lower halide chemistry of niobium, tantalum, molybdenum, and tungsten represent a category of cluster different from those so far considered in that their metal-metal bonding is best treated as involving four AO s on each metal 49, 133,144,165,178). 

Schafer, H., D. Bauer, W. Beckmann, R. Gerken, H. G. Nieder-Vahrenholz, K. J. Niehues, and H. Scholz Halides and chalcogendihalides of niobium and tantalum. Naturwissenschaften 51, 241 (1964). 

Experimentally it has been found that primary and secondary amines react by solvolysis, while only the tertiary amines generally produce reduction, if reduction is observed. It thus seemed appropriate to study the reaction of niobium (V) halides with pyridine, where proton dissociation need not be considered and any reaction would necessarily lead to a simple adduct of pyridine or reduction of the metal halide. In this work, reduction of the niobium(V) halides was observed, and the reaction products were characterized. Elucidation of the pyridine oxidation products has permitted an interpretation of the reaction mechanism in terms of the two-electron reduction of niobium(V) by the pyridine molecule. 

Preparation of Niobium(V) and Tantalum(V) Halides. The niobium(V) and tantalum (V) chloride and bromide were prepared in a system of sealed, evacuated bulbs by reacting the pure metal with gaseous halogen. For the chlorides the metal was maintained at 300° to 350° under a chlorine pressure of approximately 70 mm. of Hg the chlorine pressure was maintained By keeping the liquid chlorine immersed in a dry ice-acetone bath. For the bromides the metal was maintained at a temperature of 400° to 450° under a bromine pressure of approximately 250 mm. of Hg, maintained by leaving the liquid bromine at room temperature. 

Reactions of Tantalum(V) Chloride and Bromide with Pyridine. Table IV summarizes the reactions of niobium (V) and tantalum(V) halides with pyridine. In contrast to the niobium (V) halides, the reactions of these tantalum halides and pyridine were rapid and complete within a few minutes, and gave no evidence of reduction of the tantalum. 

From these observations it was concluded that the major products of the reduction of niobium(V) chloride with anhydrous pyridine were tetrachlorodi-(pyridine)niobium(IV) and l-(4-pyridyl)pyridinium dichloride. Oxidation-reduction titrations indicated that this reduction accounted for approximately 70% of the reaction products. In view of the rapid reaction of tantalum(V) halides with pyridine to form 1 to 1 adducts, it was assumed that the remaining 30% of niobium (V) which was not reduced was present in the reaction mixture as pentachloro(pyridine)niobium(V). On this basis the following over-all reaction is proposed ... 

Proposed Mechanisms for Reduction Reactions. Any mechanism proposed for the reduction of niobium(V) halides with pyridine must incorporate the necessary two-electron oxidation-reduction step required for the oxidation of pyridine to l-(4-pyridyl) pyridinium ion. In view of the known acid properties of the niobium(V) halides and the rapid reaction of the tantalum (V) halides to give 1 to 1 pyridine adducts, the mechanism must also include the initial coordination of pyridine to the niobium(V) halide. The reduction might then proceed through the steps shown opposite. 

The pentahalides of niobium and tantalum are predominantly covalent. Their volatility decreases from the fluorides to the iodides. Nb and Ta belong to the few elements which form stable pentaiodides. Selected physical and thermodynamic properties for the halides are listed in Table 2. All are sensitive to moist air, water or hydroxylic solvents. 

The niobium and tantalum tetrahalides are all known, except TaF4, whose non-existence is consistent with the fact that [TaF5]4 should be the least reducible halide of the group VA... 

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... 

In addition to the halides the only trivalent salt of niobium is the uncertain double sulphate (NH4)2S04.Nba(S04)3.6H80. Tantalum has not given any trivalent salts. 

---