Niobium pentavalent

Niobium Halides and Oxyhalides. AH possible haUdes of pentavalent niobium are known and preparations of lower valent haUdes generally start with the pentahaUde. Ease of reduction decreases from iodide to fluoride. 

Although niobic and tantalic acids are extremely difficult to separate, Marignac finally succeeded, not only in separating them, but also in showing that niobium is both tri- and pentavalent, whereas tantalum always has a valence of five. The separation is based on the insolubility of potassium fluotantalate in comparison with potassium fluo-oxyniobate (12, 20). In the United States the element discovered by Hatchett used to be known as columbium, but in Europe most chemists prefer to use the name niobium which Heinrich Rose gave it. 

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... 

Volumetric Methods.—Pentavalent niobium compounds differ... 

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. 

The monoxide has been variously described, according to the method of preparation, as a black powder, or glistening crystals, or beautiful black cubes. Its density varies from 6 3 to 6-7, and it is a good conductor of electricity. On being heated in air it oxidises readily to the pentoxide with considerable evolution of heat. It dissolves in hydrofluoric acid and hydrochloric acid with evolution of hydrogen, and a pentavalent niobium salt is formed in solution. It is also dissolved by boiling potassium hydroxide to form potassium niobate. This behaviour of niobium monoxide towards acids and alkali indicates that divalent salts of niobium are too unstable to exist. A divalent chloride of tantalum has, however, recently been isolated (see p. 192). 

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). 

The salts of tetravalent titanium, zirconium, and thorium and of pentavalent niobium and tantalum are diamagnetic, as would be expected, and have colors ranging from white to orange, depending upon the halogen. 

The electrodeposition of Al-Nb alloys has been reported in a BPCI-AICI3 ionic liquid containing NbCls or Nb3Clg [60]. A pentavalent niobium species is reduced... 

A number of pentavalent trispyrazolylborate derivatives (see Tris(pyrazolyl)borates) have been reported, especially for niobium. Most of them are monomeric Tp MCl2X (X = O, NR) derivatives, jj and jj -pyrazolato (16) or 1,2,4-triazolato derivatives have been prepared as potential sources of nitrides for CVD. The reactions between dilithium l,4-diaza-l,3-diene and pentachlorides have been investigated. ... 

Element 105, (eka-tantalum) should resemble tantalum and niobium, with the pentavalent state being the most important. It should exhibit several oxidation states, such as +4 and +3, in addition to the more stable +5 state. There should be an extensive range of complex ions, offering a rich chemistry. 

Among the pentavalent elements, the most important are niobium and tantalum. Niobium is an excellent material for surface treatment of steel materials for chemical industry due to its high hardness and corrosion-resistance in wet acidic conditions. Nowadays, niobium is also used for the preparation of superconductor tapes and it is used in other branches of industry, for instance in nuclear technology and metallurgy. Tantalum is also of similar importance. For these applications, it is necessary to prepare high purity metal. Molten salt electrolysis, as an alternative process to classical thermal reduction, provides niobium and tantalum with required quality. In order to optimize these processes, it is necessary to know details of both complex formation and redox chemistry of the species present in the melts. 

Reports on the marine chemistry of niobium (Nb) and tantalum (Ta) are limited to one study in the Pacific Ocean. Niobium and tantalum are pentavalent metals that are predicted to exist in sea water either as hydroxides (Nb(OH)5, Nb(OH)g, Ta(OH)5 and Ta(OH)g ) or possibly as oxyacids, similar to molybdenum and tungsten. Their distributions are not conservative like those of molybdenum and tungsten, but they may not be as particle-reactive as the other hydroxide-dominated species discussed. Dissolved niobium is low in the surface (3.0pmolkg ) and... 

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