Tungsten and Selenium

As with sulfur, tungsten forms two binary compoimds WSe2 and WSes. 

PR By direct reaction of the elements at elevated temperature in an inert atmosphere. By thermal dissociation of VSe3. 

CP Only barely dissociated between 700 °C and 800 °C in vacuo, but at higher temperature rapid dissociation. 

PR Sodium tungstate solution is saturated with H2Se, whereby selenotungstate ions [WSe4] are formed which are subsequently decomposed to WSes by acidification. PP Black, amorphous powder. 

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Graentzdoerffer A, D Rauh, A Pich, JR Andreesen (2003) Molecular and biochemical characterization of two tungsten-and selenium-containing formate dehydrogenases from Eubacterium acidamophilum that are associated with components of an iron-only hydogenase. Arch Microbiol 179 116-130. 

Fiirnsinn C, Englisch R, Ebner K, et al. 1996. Insulin-like vs. non-insulin-like stimulation of glucose metabolism by vanadium, tungsten, and selenium compounds in rat muscle. Life Sci 59(23) 1989-2000. 

Antimony, arsenic, selenium, tellurium, iridium, iron, molybdenum, osmium, potassium, rhodium, tungsten (and when primed with charcoal,) aluminium, copper, lead, magnesium, silver, tin, zinc. Interaction of lithium or calcium with chlorine tri- or penta-fluorides is hypergolic and particularly energetic. 

The first biochemical analysis of a selenium-containing XDH was reported in 1999 by Andreesen s group. This preparation was specific for xanthine and did not hydroxylate nicotinic acid. Moreover, the enzyme contained FAD, acid-labile sulfur, iron, and a dinucleotide molybdenum cofactor. Most intriguing was the near-equimolar presence of tungsten and molybdenum. It should be noted that the culture medium contained nearly equimolar levels of these metals, making one wonder whether the specificity of this enzyme for metal may be relaxed (i.e., can use Mo or W). Selenium was also found in the preparation and could be released by treatment with cyanide indicating it was also a labile cofactor. This further confirmed the chemical nature of the cofactor from the NAH enzyme from the same strain. ... 

The majority of the metals react with selenium oxybromide with the production of the corresponding metal bromides, selenium dioxide and selenium monobromide. Zinc, cadmium, chromium, nickel, cobalt, tungsten and tantalum, however, are not attacked even after prolonged heating at 100° C. 

The Oxy-Acids of the Halogens Perchlorates and Periodates Chlorates, Bromates, and lodates Chlorites Hypochlorites, Hypobromltes, and Hypolodites—Acids and Salts of Sulphur, Selenium, and Tellurium of Molybdenlum, Tungsten, and Uranium—Perchromates, Persulpbates, Perborates, and Percarbonates. 

Selenium and tellurium form similar compounds and so also do molybdenum, tungsten, and uranium, as well as chromium. Molybdyl, tungstyl, and uranyl chlorides... 

Fig. 13 is a drawing of electron-domain models of some Group VI hexafluorides. Open circles represent the electron-pairs of four of the six bonds to fluorine atoms in a Lewis, single-bond formulation of these molecules. Solid circles represent the atomic cores of oxygen, sulfur, selenium, tellurium, tungsten, and uranium (core radii, in hundreths of A, 9, 29, 42, 56, 62, and 80 2>, respectively). These hexafluorides are, in order, non-existent, extra-ordinarily unreactive, hydrolyzed slowly, hydrolyzed completely at room temperature in 24 hours, hydrolyzed readily, and hydrolyzed very rapidly. 

The older literature on molybdenum-sulfur chemistry describes several compounds6 that have later been shown to be either incorrectly formulated or non-existent. In recent times, the chemistry of molybdenum-sulfur compounds has been elucidated by modern techniques, both in solution and in the solid state. It is therefore the purpose of the present chapter to review and systematize the best information available in this area. Although the primary emphasis has been placed on molybdenum-sulfur species, for completeness and correlation purposes, certain tungsten or selenium analogs have also been included. 

Reaction (a) is fairly rapid reaction (b) is a slow one, but is enormously accelerated by traces of copper. If the reaction is carried out in the presence of a thiocyanate, which serves as an indicator for the presence of iron(III) ions and also retards reaction (b), then the reaction velocity, which is proportional to the time taken for complete decolourization, may be employed for detecting minute amounts of copper(II) ions. Tungsten and, to a lesser extent, selenium cause a catalytic acceleration similar to that of copper they should therefore be absent. 

Interactions Overabundance of one trace element can interfere with the metabolic use of another element available at normal levels. For example, addition of large amounts of zinc to a diet interferes with (antagonizes) intestinal copper absorption, resulting in copper deficiency from a diet with adequate copper content. Copper deficiency can provoke iron deficiency and anaemia. Molybdenum deficiency in animals can be induced by co-administration of large amounts of the similar element tungsten. Iron deficiency can also increase retention of cadmium and lead, and selenium has been proposed to protect against cadmium and mercury toxicity. 

N. Bartlett and P. L. Robinson, Fluorination of the Trioxides of Chromium, Molybdenum, Tungsten and Uranium by Selenium Tetrafluoride, J. Chem. Soc. (1961) 3549-3550. 

Many molybdenum and tungsten complexes have been synthesized to date, mimicking oxygen, sulfur and selenium transfer reactions. Studying their kinetic characteristics can be undertaken via several alternative routes and procedures. These methodologies and their respective results are presented in the following parts of this chapter. 

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