Niobium oxide halides

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. 

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. 

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. 

A growing chemistry of niobium oxide clusters continues to emerge. The reduced niobates and the NbO suboxide are also built up from condensation of Me units through shared vertices, namely, Nb60i2. Although the inner halides within the [MeXi2] + core are relatively inert to substitution, Nbe and Tae oxohalides, such as [Nb6Cli2-mOm]"+ m = 1 —4) and... 

Niobium and tantalum, though metallic in many respects, have chemistries in the V oxidation state that are very similar to those of typical non-metals. They have virtually no cationic chemistry but form numerous anionic species. Their halides and oxide halides, which are their most important simple compounds, are mostly volatile and are readily hydrolyzed. In their lower oxidation states they form an extraordinarily large number of metal-atom cluster compounds. Only niobium forms lower states in aqueous solution. The oxidation states and stereochemistries (excluding those in the cluster compounds) are summarized in Table 26-B-l. 

The data concerning the oxidation state of niobium and the coordination properties of its species in molten halides are incomplete and often contradictory. There is no doubt about the existence of niobium(IV) and (V) species in molten niobium-containing alkali chloride-based mixtures. The only question concerns the stability of NbClg" complex ions under an inert atmosphere. The other disputed moment involves the value of the lowest niobium oxidation state stable in chloride melts. According to the different points of view niobium-containing melts held in contact with the metal can contain Nb ", Nb + or Nb" + ions [1]. 

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. 

Niobium and Ta also form a number of polynuclear halides in which the metal has non-integral oxidation states (see text). 

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 reactions of 1,2,5-triphenylphosphole (tpp)(46), 1,2,5-triphenylphosphole oxide (tppO), sulphide (tppS), and selenide (tppSe), with niobium(v) and tan-talum(v) halides in dry, oxygen-free organic solvents have been studied. The complexes formed were characterized by i.r. and X-ray spectroscopy. The... 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

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 ... 

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... 

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 pentavalent halides and oxyhalides, as in the case of other niobium compounds, are the most stable. It is remarkable that the pentavalency is maintained with increase in the atomic weight of the halogen. All the halogen compounds are characterised by their ready tendency to undergo hydrolysis on the addition of water or even in damp air with precipitation of niobie acid and formation of the hydrogen halide. Their preparation can, therefore, be effected only in the dry way (a) synthetically, or (b) by the action of chlorine, carbon tetrachloride, or sulphur monochloride on the oxide or sulphide. They do not possess saline properties, and cannot be prepared by the action of the halogen acids on the oxide. 

Acetyl ligands, in niobium complexes, C-H BDEs, 1, 298 Achiral phosphines, on polymer-supported peptides, 12, 698 Acid halides, indium compound reactions, 9, 683 Acidity, one-electron oxidized metal hydrides, 1, 294 Acid leaching, in organometallic stability studies, 12, 612 Acid-platinum rf-monoalkynes, interactions, 8, 641 Acrylate, polymerization with aluminum catalysts, 3, 280 Acrylic monomers, lanthanide-catalyzed polymerization,... 

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