Of niobium compounds

The alloy niobium titanium (NbTi) and the intermetaUic compound of niobium and tin (Nb.3 Sn) are the most technologically advanced LTS materials presently available. Even though NbTi has a lower critical field and critical current density, it is often selected because its metallurgical properties favor convenient wire fabrication. In contrast, Nb.3Sn is a veiy brittle material and requires wire fabrication under very well-defined temperature conditions. 

Attempts to obtain fluoride compounds of niobium and tantalum with alkali earth and some transitional metals were made as early as one hundred years ago, but synthesis and identification methods were described only at later times. 

Niobium dioxyfluoride, Nb02F, and tantalum dioxyfluoride, Ta02F, can be successfully used as precursors for the synthesis of many oxyfluoride compounds of niobium and tantalum. Systematic investigations performed on MeC>2F - M2CO3 systems, in which Me = Nb or Ta and M = alkali metal, provided necessary information on optimal synthesis procedures and imparted some conformity on the mechanism of the chemical interaction between the components. 

Niobium pentafluoride is used in making other fluoro compounds of niobium. 

General.—The compounds of niobium are not so numerous as those of vanadium. The following oxides,1 Nb203, Nb02, Nb205, are known, but only the pentoxide gives rise to salts, viz. the niobates.2 The acid character of niobium pentoxide or niobic acid is very weak the niobates are decomposed, for instance, by carbon dioxide, and are readily hydrolysed to the pentoxide. Niobic acid is, in fact, very comparable in its method of preparation and behaviour to silicic acid and stannic acid. 

Definite compounds of niobium and iodine are unknown, although tantalum pentiodide and vanadium tri-iodide have been prepared. 

Niobium Pentafluoride, NbFs, is the only known compound of niobium and fluorine, and even this cannot be obtained in the free state by a wet method because of the extreme readiness with which it hydrolyses. Niobium pentoxide dissolves readily in hydrofluoric acid, but evaporation of the solution leaves a residue of the unchanged oxide. Niobium pentafluoride has been prepared synthetically 1 by passing dry fluorine over the gently heated metal contained in a boat in a platinum tube. The product is freed from platinum tetrafluoride, a little of which is formed at the same time, by distillation in vacuo at 100° to 110° . An alternative method consists in treating niobium pentachloride with anhydrous hydrogen fluoride in a freezing mixture and purifying by redistillation.3... 

Niobium Pentabromide, NbBrs.—This is the only bromine compound of niobium hitherto prepared. It is obtained by the action of bromine vapour on the coarsely powdered metal,2 or on a mixture of niobium pentoxide and carbon in the absence of air.2 In the latter case some of the oxybromide is also formed. Niobium pentabromide is a fine, purple-red powder, very similar to red phosphorus in appearance. The fused substance forms garnet-red prisms. On being strongly heated it becomes yellow, and volatilises. It melts at about 150° C., and distils undecomposed in an inert atmosphere at about 270° C. It can be distilled unchanged in an atmosphere of nitrogen or carbon dioxide. It is very hygroscopic, hydrolyses rapidly in damp air, and is decomposed by water with a hissing noise and considerable evolution of heat into niobie add and hydrobromic add. It is soluble in absolute alcohol and in dry ethyl bromide. 

Definite compounds of niobium and iodine are unknown. The preparation of an iodide from the pentabromide has been reported2 but no details are supplied. A pyridine addition compound of the pentiodide, NbIB.(C5H5N.HI)6, has been described,2 but its existence lacks confirmation.4... 

Niobium Pentoxide, Niobic Anhydride, Nb208, is probably the commonest compound of niobium. It is obtained in the treatment of niobium-bearing minerals (see p. 124), and is the starting material in the preparation of other niobium compounds. It is precipitated in the hydrated state by the hydrolysis of nearly all pentavalent niobium salts, and is formed in the gravimetric estimation of niobium (see p. 130). 

Double Fluorides of Tantalum Pentafluoride.—When solutions of tantalum pentoxide in hydrofluoric acid are treated with solutions of the fluorides of the alkali (and other) metals, double fluorides are obtained which possess the general formula R F.TaF5, where n usually varies between 1 and 3 in the most important series n=2. These double fluorides are much more stable than tantalum pentafluoride. They were among the first tantalum compounds to recdve examination,5 and still form an important class of tantalum compounds. A study of their isomorphism with the corresponding compounds of niobium... 

The most readily available pure compounds of niobium and tantalum are the pentoxides9 Nb205 and Ta205. As expected, these are more acidic than Zr02 and Hf02, dissolving readily in concentrated alkali but inappreciably in the common mineral acids (except for concentrated HF). 

G. L. Juvinall, cr-Bonded Alkyl Compounds of Niobium and Tantalum. Trimethyldi-chloroniobium and Trimethyldichlorotantalum, J. Am. Chem. Soc. 86,4202 4203 (1964). 

A different type of hydrogenation catalysts are aryloxo compounds of niobium and tantalum which are highly effective in the hydrogenation of arenes, including... 

Half-sandwich Imido Compounds of Niobium and Tantalum... 

The lack of stability of terminal oxo compounds of the heavier Group 5 metals led us naturally to the door of the isoelectronic imido (NR) ligand, where the availability of a substituent attached to the multiply-bonded group would allow both steric and electronic modulation of the products stability and reactivity. There had been a handful of half-sandwich imido complexes of the Group 5 metals synthesised by other workers, especially for vanadium and tantalum, but at that time none were known for niobium. A half-sandwich imido compound of niobium we considered, therefore, a prime target. 

This account would not be complete without mention of distortional isomerism, a term Joe Chatt was to coin in describing two isomeric forms of the six-coordinate compounds, [Mo(0)Cl2(PMe2Ph)3] (35, Figure 12) in a short communication published in Chem. Commun. in 1971, but which later was to spark an intense debate into the reality or otherwise of the phenomenon of bond stretch isomerism. We followed the debate with some interest, since we had inadvertently stumbled across seven-coordinate oxo and sulfido compounds of niobium of the type [Nb(E)Cl3(PMe3)3] (E = O or S 36) which seemed to show a closely related effect. 

Redox systems of the hexamethylbenzene cluster compounds of niobium and tantalum have been investigated, and reactions of the type... 

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