Tungsten pentavalent

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... 

Salts containing pentavalent tungsten may be obtained by the reduction of alkaU tungstate in concentrated hydrochloric acid. Salts of types M(I)2(W0C1 ) (green), M(I)(WOCl (brown-yeUow), and M(I)(WOCl 2 ) h ve been isolated. Thiocyanato and bromo salts are also known. 

Salts containing tetravalent tungsten have been prepared by various methods. The most important are the octacyanides, M(I)4(W(CN)g), which form yellow crystals and are very stable. They are isolated as salts or free acids and can be oxidized by KMnO in H2SO4 to compounds containing pentavalent tungsten, M(I)2(W(CN)g) (yellow). 

Pentavalent antimony compounds, 20 56 Pentavalent plutonium cations, 19 692 Pentavalent tungsten, 25 386 Pentavalent vanadium aqueous, 25 533... 

In keeping witii its 5d 6s2 electron configuration, tungsten forms many compounds in which its oxidation state is 6+, just as molybdenum does. It forms divalent and tetravalent compounds to about the same extent as molybdenum but its bivalent and pentavalent compounds are somewhat fewer. Its anion chemistry is closely akin to that of molybdenum. 

Among die best known simple pentavalent tungsten compounds are the pentachloride, WOE. and the pentabromide, WBr . As is true of tungsten(IV), tungsten(V) forms complexes. 

Of the rather limited number of tetravalent molybdenum and tungsten derivatives known, wre shall mention here only molybdenum disulfide (M0S2), which occurs as the ore molybdenite and the octacyano complexes Mo(CN)i 4 and W(CN) 4 which, along with their pentavalent counterparts (p. 333), represent two of the very few examples of octa-covalency. 

The corresponding hydroxide, W(OH)j, has been prepared by the electrolytic reduction of solutions of tungsten trioxide in hydrochloric or hydrofluoric acid. It is a brown powder, insoluble in sodium hydro.xide, sulphuric acid, or acetic acid, but soluble in concentrated hydrochloric acid, yielding a greenish solution which rapidly becomes blue owing to oxidation of tetravalent tungsten to the pentavalent condition. 

In pentavalent tungsten complexes the units and WO + also exist, but in... 

Volumetric methods for determining columbium are rapid and fairly satisfactory since tantajum does not interfere. But tungsten, molybdenum, and titanium must be completely removed. The methods depend on the reduction of pentavalent columbium to the trivalent condition by means of zinc, then the oxidation by standard permanganate. Taylor s method3 makes the reduction in a Jones reductor and the titration in an atmosphere of C02. Levy s method4 carries out the reduction and titration in an atmosphere of hydrogen in a conical flask from which the air is excluded. 

Trivalent uranium ion reduces water to hydrogen. Hence, stable aqueous solutions of trivalent uranium compounds cannot be prepared. Compounds of tetravalent uranium are generally similar to those of zirconium or thorium, except that some uranium compounds can be oxidized to the hexavalent form. Compounds of pentavalent uranium are of little importance because they disproportionate readily into tetravalent and hexavalent forms. The properties of hexavalent uranium are generally similar to those of hexavalent molybdenum or tungsten. In aqueous solution hexavalent uranium forms the uranyl ion UO2 ... 

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... 

CHLORO-COMPLEXES OF PENTAVALENT NIOBIUM, TANTALUM, PROTACTINIUM, TUNGSTEN AND URANIUM,... 

Ref. [17] and [18] emphasize the problem of the bivalence of niobium in molten fluorides and the necessity of a prereduction of Nb to Nb before the electrolysis in order to avoid a further dissolution of the deposited metal due to its oxidation by pentavalent niobium. The same problem is encountered with tungsten ions existing at... 

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