Selenium monochloride is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

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

chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the sy ...

with SeCl₂, SeCl₄,

chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...

, and elemental

selenium Selenium is a chemical element with the symbol Se and atomic number 34. 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, ...

. Selenium monochloride is mainly used as a

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

for the synthesis of Se-containing compounds.

Structure and properties

Selenium monochloride has the connectivity Cl-Se-Se-Cl. With a nonplanar structure, it has C₂

molecular symmetry Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of these molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can be used to predict or explain m ...

, similar to

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%� ...

and

sulfur monochloride Disulfur dichloride is the inorganic compound of sulfur and chlorine with the formula S2Cl2. Some alternative names for this compound are ''sulfur monochloride'' (the name implied by its empirical formula, SCl), ''disulphur dichloride'' (British ...

. The Se-Se

bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...

is 2.23 Å, and the Se-Cl bond lengths are 2.20 Å. The dihedral angle is 87°.

Preparation

Early routes to selenium monochloride involved

chlorination Chlorination may refer to: * Chlorination reaction In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transform ...

of elemental selenium. An improved method involves the reaction of a mixture of selenium, selenium dioxide, and hydrochloric acid:Fehér, F. "Diselenium Dichloride". In ''Handbook of Preparative Inorganic Chemistry''; Brauer, G., Ed.; Academic Press: New York, 1963; Vol. 1; p 422-433. :3 Se + SeO₂ + 4 HCl → 2 Se₂Cl₂ + 2 H₂O A dense layer of selenium monochloride settles from the reaction mixture, which can be purified by dissolving it in fuming

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

and reprecipitating it with

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

. A second method for the synthesis involves the reaction of selenium with

oleum Oleum (Latin ''oleum'', meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Ole ...

and hydrochloric acid: :2 Se + 2 SO₃ + 3 HCl → Se₂Cl₂ + H₂SO₃ + SO₂(OH)Cl The crude selenium monochloride is removed via separatory funnel. Selenium monochloride cannot be distilled without decomposition, even at reduced pressure. In

acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...

solutions, it exists in equilibrium with SeCl₂ and SeCl₄. Selenium dichloride degrades to the monochloride after a few minutes at room temperature: :3 SeCl₂ → Se₂Cl₂ + SeCl₄

Reactions

Selenium monochloride is an electrophilic selenizing agent, and thus it reacts with simple

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

s to give bis(''β''-chloroalkyl)selenide and bis(chloroalkyl)selenium dichloride. It converts

hydrazone Hydrazones are a class of organic compounds with the structure . They are related to ketones and aldehydes by the replacement of the oxygen =O with the = functional group. They are formed usually by the action of hydrazine on ketones or aldehydes. ...

s of hindered ketones into the corresponding selenoketones, the structural analogs of ketones whereby the oxygen atom is replaced with a selenium atom. Finally, the compound has been used to introduce bridging selenium ligands between the metal atoms of some iron and chromium carbonyl complexes.

References

{{Chlorides Selenium compounds Chlorides Nonmetal halides Chalcohalides