Selenide
A selenide is a chemical compound containing a selenium anion with oxidation number of −2 (Se2−), much as sulfur does in a sulfide. The chemistry of the selenides and sulfides is similar. Similar to sulfide, in aqueous solution, the selenide ion, Se2−, is prevalent only in very basic conditions. In neutral conditions, hydrogen selenide ion, HSe−, is most common. In acid conditions, hydrogen selenide, H2Se, is formed.
Some selenides are reactive to oxidation by air. Owing to the greater reducing power of selenide, metal selenides are more easily decomposed to the elements than are sulfides (tellurides are even more labile). Selenides of electropositive metals: such as aluminium selenide readily hydrolyse, even in moist air, evolving toxic hydrogen selenide gas.
Pure selenide minerals are rare, instead selenium tends to partially substitute for sulfide in many sulfide minerals. The degree of substitution is only of commercial interest for copper sulfide ores, in which case selenium is recovered as a by-product of copper refining. Some selenide minerals include ferroselite and umangite.
Polyselenide anions are chains with the composition Sen2−. Polyselenides also refer to salts of these anions. They are commonly synthesized by melting elements together in a quartz tube. Selenium and an alkali metal react to initially give white, sparingly soluble solids like monoselenides. Excess selenium leads to the formation of soluble diselenides and very soluble polyselenides with even greater amounts of selenium. Alternatively, they can be prepared by dissolving selenium and an alkali metal in a liquid ammonia. Synthesis can also be conducted in high-boiling, polar, aprotic solvents such as DMF, HMPA, and NMP. Aqueous polyselenides undergo salt metathesis with large organic counterions to form crystalline salts that are soluble in organic solvents.
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