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.
- 2 Na + n Se → Na2Sen
- Na2Sen + 2 R4NCl → (R4N)2Sen + 2 NaCl
The structures of polyselenides have been examined by X-ray crystallography. One characteristic feature of the structure is that two terminal Se-Se bonds are shorter than those bonds involving internal selenium atoms. High resolution solid state 77Se NMR spectroscopy of for [NMe4]2Se5 and [NMe4]2Se6 suggest similar confirmations of the anions (Se5)2- and (Se)2- in the solid state and in solution. The spectra of [NMe4]2Se5 show five distinct selenium sites and the [NMe4]2Se6 spectra show symmetry with only 3 crystallographically different selenium sites. Single-crystal X-ray structure determination of the two salts support the NMR data.
Polyselenides are prone to decomposition on exposure to air, in which case they are oxidized back to elemental selenium.
- Sen2− + 2 H+ + 1/2 O2 → n Se + H2O
Polyselenides form metal complexes. Sex (x=4,5,6) function as chelating ligands complexes, e.g. (C5H5)2TiSe5, which is analogous to titanocene pentasulfide. Polyselenide anions reacts with organic halides:
- 2 RX + Se22− → R2Se2 + 2 X−
Metal selenide quantum dots
Metal selenide quantum dots and nanoparticles can be prepared by a variety of synthetic methods are available, many of which require high temperatures and hazardous precursor compounds. The particles can be adapted for a variety of applications by varying the ligands coordinated to the positively-charged outer layer. Many ligand-exchange reactions are available for use, trading X,L, and Z-type ligands, the mechanism for which is still under study.
Quantum dots based on metal selenides have been extensively for their distinctive spectral properties. Core-shell alloys of cadmium sulfide and selenide are of interest in imaging and phototherapy.
- Gallium(II) selenide
- Indium(III) selenide
- Sodium selenide
- Cadmium selenide
- Zinc selenide
- Lead selenide
- Copper selenide
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