Selenides selenium compounds

Elemental selenium has been said to be practically nontoxic and is considered to be an essential trace element however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. 

Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some solid in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils. Exposure to selenium compounds (as Se) in air should not exceed 0.2 mg/m3 (8-hour time-weighted average - 40-hour week). 

Inorganic Compounds. Inorganic selenium compounds are similar to those of sulfur and tellurium. The most important inorganic compounds are the selenides, haUdes, oxides, and oxyacids. Selenium oxidation states are —2, 0, +1, +2, +4, and +6. Detailed descriptions of the compounds, techniques, and methods of preparation, and references to original work are available (1—3,5,6—10, 51—54). Some important physical properties of inorganic selenium compounds are Hsted in Table 3. 

Selenides. Selenium forms compounds with most elements. Biaary compounds of selenium with 58 metals and 8 nonmetals, and alloys with three other elements have been described (55). Most of the selenides can be prepared by a direct reaction. This reaction varies from very vigorous with alkah metals to sluggish and requiring high temperature with hydrogen. 

Selen-verbindung, /. selenium compound, -wasserstoff, m, hydrogen selenide. -was-serstoffsaure, /. hydroselenic acid, -wismut-glanz, m. guanajuatite. -zelle, /. selenium cell, -zyanid, n. selenium cyanide seleno cyanate. -zyankalium, n. potassium selenocyanate. 

Nitrogen Selenide. (Selenium Nitride). See under List of Nitride Compounds in this Vol... 

In the case of selenium compounds in the presence of water, only gold selenide, AuSe, is formed (309). 

Caution Most selenium compounds are toxic consequently care should be exercised in handling them. The hydrogen peroxide oxidation of selenides is highly exothermic, acid-catalyzed, and auto-catalytic. The procedure given for adding the hydrogen peroxide solution should be carefully followed. 

A number of zinc selenium complexes have now been characterized, with particular interest in the formation of zinc selenide semiconductors and quantum dots. In many cases analogous structures to those observed with thiol or thiolates are recorded. 77Se NMR is frequently used in characterization, and comparison with the sulfur equivalent is relevant. Zinc selenium compounds are of particular interest as precursors for metal/selenide materials and their relevance as models for selenocysteine-containing metalloproteins. 

Ammonium potassium selenide (see Selenium and selenium compounds) Amorphous silica (see also Silica)... 

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

It is well known that the toxicity of many elements depends on the physicochemical forms they assume. So, for instance, determining the total content of a certain element in a sample is definitely not sufficient to measure its toxicity. Selenium is a case in point in small amounts this element is essential to human health. But the transition from the necessary amount (about 70pg of selenium per day for an adult) to a toxic dose (about 800 pg of selenium per day) is relatively easy. In rats, moreover, the fatal dose of Se(IV) compounds is 3.2 mg kg 1 of body mass, whereas for dimethyl selenide it is 1600 mg kg 1 of body mass. Nonorganic selenium compounds [Se(IV) and Se(VI)] are believed to be the most toxic ones, whereas in the environment selenium occurs most commonly bound to amino acids (selenomethionine and selenocysteine). The least toxic forms seem to be the volatile methyl compounds of selenium, which are metabolites of a detoxication process. 

Selenium powder is insoluble in water the dioxide, selenous acid, and the alkali metal selenites are water soluble. Highly poisonous hydrogen selenide (offensive smell) is generated when an acid solution of a selenium compound is reduced by metals such as tin and zinc. Selenic acid reacts vigorously with water.1... 

Hydrogen selenide irritates the nose, eyes, and lung tissue, and disturbs the digestive and nervous systems. Solutions of selenium compounds may burn the skin and cause severe pain by skin absorption. Selenium dioxide dust irritates the respiratory system, eyes, and... 

Dry tests a. Selenium compounds mixed with sodium carbonate and heated upon charcoal odour of rotten horseradish. A foul odour, due to hydrogen selenide, H2Se, is obtained upon moistening the residue with a few drops of dilute hydrochloric acid. A black stain (due to Ag2Se) is produced when the moistened residue is placed in contact with a silver coin. 

Other areas of selenium chemistry, such as selenium rings, selenides, " organoselenium compounds, " and the focus of older, more comprehensive accounts of Se halide chemistry will not be covered here. Additionally, the vast environmental chemistry of selenium, for example, its aqueous and sediment speciation, plant uptake, nutritional and human health aspects, and appearance in coafl and coal-derived ash, are beyond the scope of this review. The previous reviews on Se-containing ligands, " biochemistry of selenium,as well as other relevant aspects, remain some of the most valuable sources for information on these subjects. 

The most commonly used oxidant for selenium compounds is H202. " The oxidation procedure normally involves addition of 30% H2O2 to a THF or preferably dichloromethane solution of the selenide at 0 °C. The oxidation proceeds chemoselectively and, thus, many potentially oxidizable functional groups such as alkenes, sulfides, amines, sulfoxides, tertiary alcohols, esters, lactones, nitriles and carboxylic acids remain intact. Several selenoxides including (36) and (37) were isolated by this method. ... 

Both classes of selenium compounds have either been postulated as reaction intermediates or could usually only be observed as short-lived intermediates. Allyl selenoxides (X = R), the oxidation products of allyl selenides, display fast rearrangement to allylic selenates, which are hydrolyzed to allylic alcohols under the standard reaction conditions of the oxidation. 

Importantly, it also occurs naturally in several oxidation states and is, therefore, redox sensitive. Methylation and hydride formation are important, and sulfur and iron compounds play an important role in the cycling of selenium. Microbiological volatilization of organic selenium, particularly dimethyl selenide, is known to be an important factor in the loss of selenium from some selenium-rich soils and waters (Frankenberger and Arshad, 2001 Oremland, 1994). Phytoplankton can also promote the production of gaseous selenium compounds in the marine environment (Amouroux et aL, 2001). 

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