Similar to sulfide, for an aqueous solution, 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, although the degree of substitution is only of commercial interest for copper sulfide ores. Selenium is thus recovered as a by-product of copper mining. 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−
- Bernd E. Langner "Selenium and Selenium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_525.
- Kolis, J. "Coordination Chemistry of Polychalcogen Anions and Transition Metal Carbonyls" Coordination Chemistry Reviews 1990, volume 105, pp. 195-219. doi:10.1016/0010-8545(90)80023-M
- Thompson, D.; Boudjouk, P. A :Convenient Synthesis of Alkali Metal Selenides and Diselenides in Tetrahydrofuran and the Reactivity Differences Exhibited By These Salts Toward Organic Bromides" Journal of Organic Chemistry 1988, volume 53, pp. 2109-2112. doi: 10.1021/jo00244a051
- Barrie, PJ.; Clark, R.J.H.; Selenium Solid-State NMR Spectroscopy and Structures of Tetramethylammonium Pentaselenide and Hexaselenide Complexes. Inorg. Chem, 1995, 34, 4299-4304 DOI: 10.1021/ic00121a006