Vanadium and tantalum

King and Garner, using the isotopic method made an attempt to study 

IONS OF SAME METAL IN DIFFERENT OXIDATION STATES 

Krishnamurty and Wahl tried a number of separation methods and eventually used a modified dipyridyl-ammonia separation to obtain kinetic data for this exchange. The zero-time exchange lay between 25 and 50 %, depending on the conditions the activity of the V(dipy)3 ion was measured. The rate law obtained for perchlorate media was 

The rate of exchange was found to be dependent on the hydrogen-ion concentration (up to 5 X 10 M) in a manner which led to these authors suggesting that the exchange pathways 

Addition of chloride in the range 5 x 10 to 3 x 10 M, at constant acidity and ionic strength, was found to increase the rate of exchange. This was interpreted in terms of an exchange pathway involving the VCl ion, viz. 

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Identify the element with the larger atomic radius in each of the following pairs (a) vanadium and titanium (b) silver and gold (c) vanadium and tantalum (d) rhodium and iridium. 

A.Y. Esayed, D.O. Northwood, Metal hydrides A review of group V transition metals-nio-bium, vanadium and tantalum, Int. J. Hyd. Ener. 17 (1992) 41-52. 

Symbol Nb atomic number 41 atomic weight 92.906 a Group VB (Group 5) element a transition metal in the triad of vanadium and tantalum also. 

Carotta, M. C. Guidi, V. Malagu, C. Vendemiati, B. Zanni, A. Martinelli, G. Sacerdoti, M. Licoccia, S. Vona, M. L. D. Traversa, E., Vanadium and tantalum-doped titanium oxide (TiTaV) a novel material for gas sensing, Sens. Actuators B 2000, 108, 89-96... 

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. 

The transition metal niobium attracts a lot of attention of researchers because of its relatively high superconducting transition temperature. It turned out that niobium shows a number of pronounced anomalies in the phonon dispersion, which are also typical for vanadium and tantalum. The anomalies in [00 ], [0 ] directions appear as a crossover of the longitudinal and transverse branches at = 0.7 or = 0.3 as well as additional maxima and minima. 

In the pharmaceutical industry products such as penicillin occur in fermentation mixtures that are quite complex, and liquid extraction can be used to separate the penicillin. Many metal separations are being done commercially by extraction of aqueous solutions, such as copper-iron, uranium-vanadium, and tantalum-columbium. 

R. J. H. Clark and D. Brown, The Chemisty of Vanadium, Niobium and Tantalum, Pergamon Press, Ehnsford, N. Y., 1975. 

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

The elements of Group 5 are in many ways similar to their predecessors in Group 4. They react with most non-metals, giving products which are frequently interstitial and nonstoichiometric, but they require high temperatures to do so. Their general resistance to corrosion is largely due to the formation of surface films of oxides which are particularly effective in the case of tantalum. Unless heated, tantalum is appreciably attacked only by oleum, hydrofluoric acid or, more particularly, a hydrofluoric/nitric acid mixture. Fused alkalis will also attack it. In addition to these reagents, vanadium and niobium are attacked by other hot concentrated mineral acids but are resistant to fused alkali. 

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... 

Niobium and tantalum also form various oxide phases but they are not so extensive or well characterized as those of vanadium. Their pentoxides are relatively much more stable and difficult to reduce. As they are attacked by cone HF and will dissolve in fused alkali, they may perhaps... 

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. 

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