Editing Selenium (section)

==Chemical compounds==
{{Main|Selenium compounds}}
Selenium compounds commonly exist in the [[oxidation state]]s −2, +2, +4, and +6. It is a [[nonmetal]] (more rarely considered a [[metalloid]]) with properties that are intermediate between the elements above and below in the [[periodic table]], [[sulfur]] and [[tellurium]], and also has similarities to [[arsenic]].<ref name="ge"/>

===Chalcogen compounds===
[[File:Selenium-dioxide-chain-3D-balls.png|thumb|Structure of the polymer SeO<sub>2</sub>: The (pyramidal) selenium atoms are yellow.|left]]
Selenium forms two [[oxide]]s: [[selenium dioxide]] (SeO<sub>2</sub>) and [[selenium trioxide]] (SeO<sub>3</sub>). Selenium dioxide is formed by combustion of elemental selenium:<ref name="house2008" />
{{block indent|{{chem2|Se + O2 -> SeO2}}}}
It is a [[polymer]]ic solid that forms monomeric SeO<sub>2</sub> molecules in the gas phase. It dissolves in water to form [[selenous acid]], H<sub>2</sub>SeO<sub>3</sub>. Selenous acid can also be made directly by oxidizing elemental selenium with [[nitric acid]]:<ref name="wiberg_holleman">{{cite book |title=Inorganic chemistry |last1=Wiberg |first1=Egon |last2=Wiberg |first2=Nils |last3=Holleman |first3=Arnold Frederick |publisher=Academic Press |year=2001 |location=San Diego |isbn=978-0-12-352651-9 |page=583}}</ref>

{{block indent|{{chem2|3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO}}}}
Unlike sulfur, which forms a stable [[sulfur trioxide|trioxide]], selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185&nbsp;°C:<ref name="house2008" /><ref name="wiberg_holleman" />

{{block indent|{{chem2| 2 SeO3 -> 2 SeO2 + O2}} (ΔH {{=}} −54&nbsp;kJ/mol)}}
Selenium trioxide is produced in the laboratory by the reaction of [[anhydrous]] [[potassium selenate]] (K<sub>2</sub>SeO<sub>4</sub>) and sulfur trioxide (SO<sub>3</sub>).<ref>{{Greenwood&Earnshaw|page=780}}</ref>

[[Salt (chemistry)|Salts]] of selenous acid are called selenites. These include [[silver selenite]] (Ag<sub>2</sub>SeO<sub>3</sub>) and [[sodium selenite]] (Na<sub>2</sub>SeO<sub>3</sub>).

[[Hydrogen sulfide]] reacts with aqueous selenous acid to produce [[selenium disulfide]]:
{{block indent|{{chem2|H2SeO3 + 2 H2S -> SeS2 + 3 H2O}}}}

Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS<sub>2</sub>, with individual rings varying in composition, such as Se<sub>4</sub>S<sub>4</sub> and Se<sub>2</sub>S<sub>6</sub>. Selenium disulfide has been used in shampoo as an anti[[dandruff]] agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in [[fireworks]].<ref name="wiberg_holleman" />

Selenium trioxide may be synthesized by dehydrating [[selenic acid]], H<sub>2</sub>SeO<sub>4</sub>, which is itself produced by the oxidation of selenium dioxide with [[hydrogen peroxide]]:<ref>{{Cite book |doi=10.1002/9780470132517.ch9 |last1=Seppelt |first1=K. |last2=Desmarteau |first2=Darryl D. |chapter=Selenonyl Difluoride |title=Inorganic Syntheses |year=1980 |volume=20 |pages=36–38 |isbn=978-0-471-07715-2 }} The report describes the synthesis of selenic acid.</ref>
{{block indent|{{chem2|SeO2 + H2O2 -> H2SeO4}}}}

Hot, concentrated selenic acid reacts with gold to form gold(III) selenate.<ref>{{Cite journal |last1=Lenher |first1=V. |date=April 1902 |title=Action of selenic acid on gold |journal=Journal of the American Chemical Society |volume=24 |issue=4 |pages=354–355 |doi=10.1021/ja02018a005 |bibcode=1902JAChS..24..354L |url=https://zenodo.org/record/1428902}}</ref>

===Halogen compounds===
Selenium reacts with [[fluorine]] to form [[selenium hexafluoride]]:

{{block indent|{{chem2|Se8 + 24 F2 -> 8 SeF6}}}}

In comparison with its sulfur counterpart ([[sulfur hexafluoride]]), [[selenium hexafluoride]] (SeF<sub>6</sub>) is more reactive and is a toxic [[pulmonary]] irritant.<ref>{{cite book |last1=Proctor |first1=Nick H. |title=Proctor and Hughes' chemical hazards of the workplace |last2=Hathaway |first2=Gloria J. |publisher=Wiley-IEEE |year=2004 |isbn=978-0-471-26883-3 |editor=Hughes, James P. |edition=5th |page=625}}</ref> [[Selenium tetrafluoride]] is a laboratory-scale [[fluorinating agent]].

The only stable [[chloride]]s are [[selenium tetrachloride]] (SeCl<sub>4</sub>) and [[selenium monochloride]] (Se<sub>2</sub>Cl<sub>2</sub>), which might be better known as selenium(I) chloride and is structurally analogous to [[disulfur dichloride]]. Metastable solutions of [[selenium dichloride]] can be prepared from [[sulfuryl chloride]] and selenium (reaction of the elements generates the [[Selenium tetrachloride|tetrachloride]] instead), and constitute an important reagent in the preparation of selenium compounds (e.g. Se<sub>7</sub>). The corresponding [[bromide]]s are all known, and recapitulate the same stability and structure as the chlorides.<ref>{{cite book
|title=Handbook of chalcogen chemistry: new perspectives in sulfur, selenium and tellurium |author=Xu, Zhengtao |editor=Devillanova, Francesco A. |publisher=Royal Society of Chemistry |year=2007 |isbn=978-0-85404-366-8 |page=460}}</ref>

The [[iodide]]s of selenium are not well known, and for a long time were believed not to exist.<ref name=":1">{{Cite journal |last1=Gopal |first1=Madhuban |last2=Milne |first2=John |date=October 1992 |title=Spectroscopic evidence for selenium iodides in carbon disulfide solution: Se3I2, Se2I2, and SeI2 |url=https://pubs.acs.org/doi/abs/10.1021/ic00048a017 |journal=Inorganic Chemistry |language=en |volume=31 |issue=22 |pages=4530–4533 |doi=10.1021/ic00048a017 |issn=0020-1669}}</ref> There is limited [[Spectroscopy|spectroscopic]] evidence that the lower iodides may form in bi-elemental solutions with nonpolar solvents, such as [[carbon disulfide]]<ref>{{Cite journal |last=McCullough |first=James D. |date=December 1939 |title=Evidence for Existence of a Selenium Iodide |url=https://pubs.acs.org/doi/abs/10.1021/ja01267a052 |journal=Journal of the American Chemical Society |language=en |volume=61 |issue=12 |pages=3401–3402 |doi=10.1021/ja01267a052 |bibcode=1939JAChS..61.3401M |issn=0002-7863}}</ref> and [[carbon tetrachloride]];<ref name=":1" /> but even these appear to [[Photosensitivity|decompose under illumination]].<ref>Rao, M.&nbsp;R.&nbsp;Aswatha&nbsp;Narayana. [https://www.ias.ac.in/public/Volumes/seca/012/04/0410-0415.pdf "Selenium iodide"]. In ''Proceedings of the Indian Academy of Sciences-Section&nbsp;A'', vol.&nbsp;12, pp.&nbsp;410-415. Springer India, 1940.</ref>

Some selenium oxyhalides—[[seleninyl fluoride]] (SeOF<sub>2</sub>) and [[selenium oxychloride]] (SeOCl<sub>2</sub>)—have been used as specialty solvents.<ref name="house2008" />

===Metal selenides===
[[File:Polyselenide.svg|thumb|Structures of two polyselenide anions<ref>{{Greenwood&Earnshaw2nd|page=763-765}}</ref>]]
Analogous to the behavior of other chalcogens, selenium forms [[hydrogen selenide]], H<sub>2</sub>Se. It is a strongly [[wikt:odiferous|odiferous]], toxic, and colorless gas. It is more acidic than H<sub>2</sub>S. In solution it ionizes to HSe<sup>−</sup>. The selenide dianion Se<sup>2−</sup> forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include [[mercury selenide]] (HgSe), [[lead selenide]] (PbSe), [[zinc selenide]] (ZnSe), and [[copper indium gallium diselenide]] (Cu(Ga,In)Se<sub>2</sub>). These materials are [[semiconductor]]s. With highly electropositive metals, such as [[aluminium]], these selenides are prone to hydrolysis, which may be described by this idealized equation:<ref name="house2008" />

:{{chem2 | Al2Se3 + 6 H2O -> 2 Al(OH)3 + 3 H2Se }}

Alkali metal selenides react with selenium to form polyselenides, {{chem|Se|n|2-}}, which exist as chains and rings.

===Other compounds===
Tetraselenium tetranitride, Se<sub>4</sub>N<sub>4</sub>, is an explosive orange compound analogous to [[tetrasulfur tetranitride]] (S<sub>4</sub>N<sub>4</sub>).<ref name="house2008" /><ref>{{cite journal |last1=Woollins |first1=Derek |title=The Reactivity of Se<sub>4</sub>N<sub>4</sub> in Liquid Ammonia |journal=[[Polyhedron (journal)|Polyhedron]] |year=1993 |volume=12 |pages=1129–1133 |doi=10.1016/S0277-5387(00)88201-7 |issue=10 |last2=Kelly |first2=Paul F.}}</ref><ref>{{cite journal |last1=Kelly |first1=P.F. |last2=Slawin |first2=A.M.Z. |last3=Soriano-Rama |first3=A. |title=Use of Se<sub>4</sub>N<sub>4</sub> and Se(NSO)<sub>2</sub> in the preparation of palladium adducts of diselenium dinitride, Se<sub>2</sub>N<sub>2</sub>; crystal structure of {{chem|[PPh|4|]|2|[Pd|2|Br|6|(Se|2|N|2|)|]}} |journal=[[Dalton Transactions]] |date=1997 |pages=559–562 |doi=10.1039/a606311j |issue=4}}</ref> It can be synthesized by the reaction of [[selenium tetrachloride]] (SeCl<sub>4</sub>) with [[Metal bis(trimethylsilyl)amides|{{chem|[((CH|3|)|3|Si)|2|N]|2|Se}}]].<ref>{{cite journal |doi=10.1021/ic00060a031 |title=A simple, efficient synthesis of tetraselenium tetranitride |year=1993 |last1=Siivari |first1=Jari |last2=Chivers |first2=Tristram |last3=Laitinen |first3=Risto S. |journal=Inorganic Chemistry |volume=32 |issue=8 |pages=1519–1520}}</ref>

Selenium reacts with [[cyanide]]s to yield [[Potassium selenocyanate|selenocyanates]]:<ref name="house2008" />

:{{chem2 | 8 KCN + Se8 -> 8 KSeCN }}

===Organoselenium compounds===
{{Main|Organoselenium chemistry}}
Selenium, especially in the II oxidation state, forms a variety of organic derivatives. They are structurally analogous to the corresponding [[organosulfur compounds]]. Especially common are selenides (R<sub>2</sub>Se, analogues of [[thioether]]s), diselenides (R<sub>2</sub>Se<sub>2</sub>, analogues of [[disulfide]]s), and [[selenol]]s (RSeH, analogues of [[thiol]]s). Representatives of selenides, diselenides, and selenols include respectively [[selenomethionine]], [[diphenyldiselenide]], and [[benzeneselenol]]. The [[sulfoxide]] in sulfur chemistry is represented in selenium chemistry by the selenoxides (formula RSe(O)R), which are intermediates in organic synthesis, as illustrated by the [[selenoxide elimination]] reaction. Consistent with trends indicated by the [[double bond rule]], selenoketones, R(C=Se)R, and selenaldehydes, R(C=Se)H, are rarely observed.<ref>{{cite journal |last1=Erker |first1=G. |last2=Hock |last3=Krüger |last4=Werner |last5=Klärner |last6=Artschwager-Perl |title=Synthesis and Cycloadditions of Monomeric Selenobenzophenone |journal=Angewandte Chemie International Edition in English |volume=29 |issue=9 |pages=1067–1068 |year=1990 |doi=10.1002/anie.199010671 |first2=R. |first3=C. |first4=S. |first5=F.G. |first6=U.}}</ref>