Tungsten hexavalent

Some metals used as metallic coatings are considered nontoxic, such as aluminum, magnesium, iron, tin, indium, molybdenum, tungsten, titanium, tantalum, niobium, bismuth, and the precious metals such as gold, platinum, rhodium, and palladium. However, some of the most important poUutants are metallic contaminants of these metals. Metals that can be bioconcentrated to harmful levels, especially in predators at the top of the food chain, such as mercury, cadmium, and lead are especially problematic. Other metals such as silver, copper, nickel, zinc, and chromium in the hexavalent oxidation state are highly toxic to aquatic Hfe (37,57—60). 

Anionic Complexes. Compounds of tungsten with acid anions other than haUdes and oxyhaUdes are relatively few in number, and are known only in the form of complex salts. A number of salts containing hexavalent tungsten are known. Potassium octafluorotungstate [57300-87-5] K WFg, can be prepared by the action of KI on W(CO)g in an IF medium. The addition of tungstates to aqueous hydrofluoric acid gives salts that are mostly of the type M(I)2(W2F. Similarly, double salts of tungsten oxydichloride are known. 

The route to carbene initiation for systems catalyzed solely by transition metal salts (55, 54), or their combinations with Lewis acids such as A1C13 (55), is not well established. Nevertheless, some evidence suggests reduction of the metal by the olefinic substrate (55). Zero-valent (56) and hexavalent (57, 55) tungsten complexes that promote metathesis when activated by UV radiation are the least-understood metathesis systems. 

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... 

By far the greatest number of tungsten compounds are those in which the element is hexavalent. These include all common halides except the iodide, i.e.. WF. WCl, as well as a number of oxvhalrdes, WOF4, WOCI4, WO Cl , WOBr4, and WO Br2, the Irioxide. trisullide, dibonde, and diphosphide already mentioned, various complexes and organometallic compounds, and the anions. 

Polyoxometalate anions, especially those of hexavalent molybdenum and tungsten interact with rare-earth cations to generate a seemingly endless variety of complexes in which the rare earths function as encrypted cations, as core heteroatoms, or as linkers of polyoxometalate fragments to yield discrete anionic assemblies currently incorporating as few as 8 and as many as 164 metal centers,... 

Hexavalent uranium is far more easily reduced than either hexavalent molybdenum or tungsten the potential for the UO —> U+4 couple is close to that for the Cu2+ —> Cu couple. 

The compounds known as tungsten bronzes are reduction products of the tungstates of the alkali and alkaline earth metals. Their exact constitution is not known, but it is generally recognised that the molecule contains several hexavalent tungsten atonrs and one tetravalent tungsten atom, and may be represented by the formula R 20.(W03)4,. WOj. The empirical formula accordingly becomes R 2(W03),.j,4. 

In contrast, hexavalent transition metals, such as molybdenum and tungsten, polymerize in acid by aggregation to specifically favored geometries containing oxide octahedra of MoOg and W06 (the iso poly acids) (5a). These oxide structures have cavities which accept foreign ions, to form heteropoly ions (5b). The central cavity is a tetrahedral site, often occupied by P0, Si0, etc, a fact utilized in phosphate precipitation ... 

Among the hexavalent elements, molybdenum and tungsten seem to be the most technologically important metals because of their use as deposits on steels in the form of protecting metal or metal boride layers. Since the main features of molybdenum and tungsten electrolytes are similar, the following text will deal with the properties of the former. 

Metallic tungsten as an anode in basic solutions is dissolved electrolytically (oxidized to the hexavalent state). The six-electron stoichiometry of the overall reaction... 

The oxidation of tungsten in scrap material to the hexavalent state by heating in air or oxygen-eruiched air, because only hexavalent tungsten dissolves in an alkaline leach process. 

Electrolysis [5.30]. The principle of the method is to have the tungsten scnq> as anode in an electrolyte like sodium hydroxide or sodium carbonate aqueous solution. The tungsten atoms are oxidized electrolytically to the hexavalent state. The resulting solution contains sodium tungstate besides insoluble oxides or oxide hydrates of flie other constituents. Rotating drum or disk electrolytic cells are the applied equipment. 

---