Tantalum forming

Niobium-Tantalum Niobium and tantalum form solid-solution alloys which are resistant to many corrosive media and possess all the valuable properties of the pure metals. This could have great practical value since in a number of branches of technology it might permit the replacement of pure tantalum by a cheaper alloy of niobium and tantalum. Miller" and Argent" reported data on the resistance of the niobium-tantalum system, but the tests were only carried out under mild conditions and the data have only limited significance. However, Gulyaev and Georgieva and Kieffer, Bach and Slempkowski carried out tests at elevated temperatures and their work indicated that the corrosion rates of the alloys are substantially that of tantalum provided the niobium content does not exceed 50%. 

The first comprehensive investigation of the TaF5 - HF - H2O system was performed by Buslaev and Nikolaev [292]. Based on the analysis of solubility isotherms, and on conductometric and potentiometric titrations, the authors concluded that in this solution, tantalum forms oxyfluorotantalic acid, H2TaOF5, similar to the formation of H NbOFs in solutions containing NbF5. 

An analysis of the melting diagram led to the conclusion that, in fluoride and fluoride-chloride melts, tantalum forms the complex ions TaFg3 or TaF7Cl3, respectively [37,306]. 

Hexavalent molybdenum and tungsten, pentavalent vanadium and, to a more limited extent, niobium and tantalum form a very large number of polyoxoanions ( heteropolyanions such as [PW12O40]3- and isopolyanions such as [M07O24]6-). Unlike the polyoxoanions of the post transition elements the heteropolyanions for the most part are discrete, compact species of high... 

The second class of hexanuclear clusters also contains an octahedron of metal atoms, but they are coordinated by twelve halide ligands along the edges (Fig. 16.64b). Niobium and tantalum form clusters of this type. Here the bonding situation is somewhat more complicated The metal atoms are surrounded by a very distorted square prism of (bur metal and four halogen atoms. Furthermore, these compounds are electron deficient in the same sense as the boranes—there are fewer pairs of electrons than orbitals to receive them and so fractional bond orders of are obtained. 

The chemistry of hydrido complexes of group V metals seems to reflect the usual tendency for vanadium to behave differently from the other two elements, although generalizations are probably premature in such a new field. Certainly niobium and tantalum form numerous hydrides of similar composition and properties, appearing to have no vanadium counterparts to date, but there have been no systematic investigations involving all three elements under comparable conditions, as is clear from the following discussions. 

Niobium and tantalum form stable fluoride complexes which are extractable with oxygen-containing solvents and can thus be separated from many metals, such as Ti, Zr, Sn, Mo, W, U, and Fe [6,7]. By suitable choice of concentrations of HF, HCl, and H2SO4, tantalum can be separated from niobium and vice versa [6]. MIBK is most often used as the solvent. Niobium and Ta can be stripped from the organic phase with a H2O2 solution. 

In dilute HF medium, tantalum forms a complex, TaFe", which combines with the basic dye Methyl Violet to form an ion-pair, which can be extracted with benzene. The coloured extract has been the basis for a sensitive method of determining Ta [90]. 

Metal complexes have a variety of stractures. Silver complexes are often linear beryllium complexes are usually tetrahedral iron forms a carbonyl compound that has a trigonal bipyramidal structure cobalt(lll) complexes are octahedral and tantalum forms an eight-coordinated fluoride complex (Figure 3.1). Although a variety of coordination numbers and structures have been observed in metal complexes, the only common coordination numbers are four and six the common structures corresponding to these coordination numbers are tetrahedral and square planar, and octahedral, respectively. In studying metal complexes, it soon becomes clear that the octahedral structure is by far the most common of these configurations. 

Niobium and tantalum form orange to orange-red chelates with pyrogallol-4-sulfonic acid, even in acid solutions. In the presence of citric acid tantalum does not form a chelate. 

Catechol violet (I) forms colored complexes with many ions but only the complexes with boric acid, germanium, niobium and tantalum are unaffected by EDTA. At pH 6-7 boric acid forms a reddish complex, germanium purple and niobium and tantalum blue-violet. At pH 1 and in the presence of sufficient EDTA, only tantalum forms a colored complex. ... 

With pyrogallol-4-sulfonic acid, tantalum forms a yellow 1 1 complex. This color reaction of tantalum is highly selective at pH 1.5 and in the presence of oxalic acid. Niobium (compare page 332) does not react with this reagent under these conditions. 

When exposed to oxidizing or slightly anodic conditions, tantalum forms a thin impervious layer of tantalum oxide. This passivating oxide has the broadest range of stability with regard to chemical attack or thermal breakdown compared to other metallic films. Chemicals or conditions that attack tantalum, such as hydrofluoric acid, are those which penetrate or dissolve the film. 

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