Selenium compounds
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Selenium compounds commonly exist in the
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s −2, +2, +4, and +6.


Chalcogen compounds

Selenium forms two
oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s: selenium dioxide (SeO2) and
selenium trioxide Selenium trioxide is the inorganic compound with the formula Se O3. It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid. It is of academic interest as a precursor to Se(VI) compounds.Egon Wiberg, Arnold Frederick Holl ...
(SeO3). Selenium dioxide is formed by the reaction of elemental selenium with oxygen: :Se8 + 8 O2 -> 8 SeO2 It is a
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
ic solid that forms monomeric SeO2 molecules in the gas phase. It dissolves in water to form
selenous acid Selenous acid (or selenious acid) is the chemical compound with the formula . Structurally, it is more accurately described by . It is the principal oxoacid of selenium; the other being selenic acid. Formation and properties Selenous acid is ana ...
, H2SeO3. Selenous acid can also be made directly by oxidizing elemental selenium with
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
: :3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 Â°C: :2 SeO3 -> 2 SeO2 + O2 (ΔH = −54 kJ/mol) Selenium trioxide is produced in the laboratory by the reaction of
anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
potassium selenate Potassium selenate, , is an odorless, white solid that forms as the potassium salt of selenic acid. Preparation Potassium selenate is produced by the reaction of selenium trioxide and potassium hydroxide. : Alternatively, it can be made by tr ...
(K2SeO4) and sulfur trioxide (SO3). Salts of selenous acid are called selenites. These include
silver selenite Silver selenite is an inorganic compound of formula Ag2SeO3. Production Silver selenite is formed during the recovery of selenium from copper anode slimes when they are subjected to oxidative roasting, causing some silver selenide to be converted ...
(Ag2SeO3) and sodium selenite (Na2SeO3).
Hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
reacts with aqueous selenous acid to produce
selenium disulfide Selenium disulfide, also known as selenium sulfide, is a chemical compound and medication used to treat seborrheic dermatitis, dandruff, and pityriasis versicolor. It is applied to the affected area as a lotion or shampoo. Symptoms frequently ret ...
: :H2SeO3 + 2 H2S -> SeS2 + 3 H2O Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS2, with individual rings varying in composition, such as Se4S4 and Se2S6. Selenium disulfide has been used in shampoo as an anti
dandruff Dandruff is a skin condition that mainly affects the scalp. Symptoms include flaking and sometimes mild itchiness. It can result in social or self-esteem problems. A more severe form of the condition, which includes inflammation of the skin, ...
agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in
fireworks Fireworks are a class of Explosive, low explosive Pyrotechnics, pyrotechnic devices used for aesthetic and entertainment purposes. They are most commonly used in fireworks displays (also called a fireworks show or pyrotechnics), combining a l ...
. Selenium trioxide may be synthesized by dehydrating
selenic acid Selenic acid is the inorganic compound with the formula . It is an oxoacid of selenium, and its structure is more accurately described as . It is a colorless compound. Although it has few uses, its derivative sodium selenate is used in the prod ...
, H2SeO4, which is itself produced by the oxidation of selenium dioxide with
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%†...
: :SeO2 + H2O2 -> H2SeO4 Hot, concentrated selenic acid can react with gold to form gold(III) selenate.


Halogen compounds

Iodides of selenium are not well known. The only stable
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
is
selenium monochloride Selenium monochloride is an inorganic compound with the formula Se2Cl2. Although it is called selenium monochloride, a more descriptive name might be diselenium dichloride. It is a reddish-brown, oily liquid that hydrolyses slowly. It exists in c ...
(Se2Cl2), which might be better known as selenium(I) chloride; the corresponding bromide is also known. These species are structurally analogous to the corresponding
disulfur dichloride Disulfur dichloride is the inorganic compound of sulfur and chlorine with the Chemical formula, formula S2Cl2. Some alternative names for this compound are ''sulfur monochloride'' (the name implied by its empirical formula, SCl), ''disulphur dich ...
. Selenium dichloride is an important reagent in the preparation of selenium compounds (e.g. the preparation of Se7). It is prepared by treating selenium with sulfuryl chloride (SO2Cl2). Selenium reacts with
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
to form
selenium hexafluoride Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a very toxic colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications. Structure, preparation, and reacti ...
: :Se8 + 24 F2 -> 8 SeF6 In comparison with its sulfur counterpart (
sulfur hexafluoride Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non- flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached ...
),
selenium hexafluoride Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a very toxic colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications. Structure, preparation, and reacti ...
(SeF6) is more reactive and is a toxic pulmonary irritant. Some of the selenium oxyhalides, such as selenium oxyfluoride (SeOF2) and
selenium oxychloride Selenium oxydichloride is the inorganic compound with the formula SeOCl2. It is a colorless liquid. With a high dielectric constant (55) and high specific conductance, it is an attractive solvent. Structurally, it is a close chemical relative of ...
(SeOCl2) have been used as specialty solvents.


Selenides

Analogous to the behavior of other chalcogens, selenium forms
hydrogen selenide Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic sele ...
, H2Se. It is a strongly odiferous, toxic, and colorless gas. It is more acidic than H2S. In solution it ionizes to HSe−. The selenide dianion Se2− forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include
mercury selenide Mercury selenide (HgSe; sometimes mercury(II) selenide) is a chemical compound of mercury and selenium. It is a grey-black crystalline solid semi-metal with a sphalerite structure. The lattice constant is 0.608 nm. HgSe occurs naturally as ...
(HgSe),
lead selenide Lead selenide (PbSe), or lead(II) selenide, a selenide of lead, is a semiconductor material. It forms cubic crystals of the NaCl structure; it has a direct bandgap of 0.27 eV at room temperature. (Note that incorrectly identifies PbSe and o ...
(PbSe),
zinc selenide Zinc selenide (ZnSe) is a light-yellow, solid compound comprising zinc (Zn) and selenium (Se). It is an intrinsic semiconductor with a band gap of about 2.70  eV at . ZnSe rarely occurs in nature, and is found in the mineral that was named af ...
(ZnSe), and
copper indium gallium diselenide Copper indium gallium (di)selenide (CIGS) is a I- III- VI2 semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide ...
(Cu(Ga,In)Se2). These materials are
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
s. With highly electropositive metals, such as
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, these selenides are prone to hydrolysis: :Al2Se3 + 3 H2O -> Al2O3 + 3 H2Se Alkali metal selenides react with selenium to form polyselenides, , which exist as chains.


Other compounds

Tetraselenium tetranitride, Se4N4, is an explosive orange compound analogous to tetrasulfur tetranitride (S4N4). It can be synthesized by the reaction of
selenium tetrachloride Selenium tetrachloride is the inorganic compound composed with the formula SeCl4. This compound exists as yellow to white volatile solid. It is one of two commonly available selenium chlorides, the other example being selenium monochloride, Se2 ...
(SeCl4) with . Selenium reacts with
cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
s to yield selenocyanates: :8 KCN + Se8 -> 8 KSeCN


Organoselenium compounds

Selenium, especially in the II oxidation state, forms stable bonds to
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
, which are structurally analogous to the corresponding
organosulfur compounds Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur ...
. Especially common are selenides (R2Se, analogues of thioethers), diselenides (R2Se2, analogues of disulfides), and selenols (RSeH, analogues of
thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...
s). Representatives of selenides, diselenides, and selenols include respectively selenomethionine,
diphenyldiselenide Diphenyl diselenide is the chemical compound with the formula (C6H5)2Se2, abbreviated Ph2Se2. This orange-coloured solid is the oxidized derivative of benzeneselenol. It is used as a source of the PhSe unit in organic synthesis. Preparation ...
, and benzeneselenol. The
sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
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 Selenoxide elimination (also called α-selenation) is a method for the chemical synthesis of alkenes from selenoxides. It is most commonly used to synthesize α,β-unsaturated carbonyl compounds from the corresponding saturated analogues. It is me ...
reaction. Consistent with trends indicated by the
double bond rule In chemistry, the double bond rule states that elements with a principal quantum number greater than 2 for their valence electrons (period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double bon ...
, selenoketones, R(C=Se)R, and selenaldehydes, R(C=Se)H, are rarely observed.


See also

*
Arsenic compounds Compounds of arsenic resemble in some respects those of phosphorus which occupies the same group (column) of the periodic table. The most common oxidation states for arsenic are: −3 in the arsenides, which are alloy-like intermetallic compound ...
* Bromine compounds


References

{{Selenium compounds Selenium Selenium compounds Chemical compounds by element