Selenium monochloride

Selenium monochloride or diselenium dichloride is an inorganic compound with the formula Se2Cl2. Although a common name for the compound is selenium monochloride, reflecting its empirical formula, IUPAC does not recommend that name, instead preferring the more descriptive diselenium dichloride.

Diselenium dichloride is a reddish-brown, oily liquid that hydrolyses slowly. It exists in chemical equilibrium with SeCl2, SeCl4, chlorine, and elemental selenium. Diselenium dichloride is mainly used as a reagent for the synthesis of Se-containing compounds.

Structure and properties
Dielenium dichloride has the connectivity Cl\sSe\sSe\sCl. With a nonplanar structure, it has C2 molecular symmetry, similar to hydrogen peroxide and disulfur dichloride, which is referred to as gauche. The Se-Se bond length is 223 pm, and the Se-Cl bond lengths are 220 pm. The dihedral angle between the Cl^{a}\sSe\sSe and Se\sSe\sCl^{b} planes is 87°.

Preparation
Early routes to diselenium dichloride involved chlorination of elemental selenium. An improved method involves the reaction of a mixture of selenium, selenium dioxide, and hydrochloric acid:
 * 3 Se + SeO2 + 4 HCl → 2 Se2Cl2 + 2 H2O

A dense layer of diselenium dichloride settles from the reaction mixture, which can be purified by dissolving it in fuming sulfuric acid and reprecipitating it with hydrochloric acid. A second method for the synthesis involves the reaction of selenium with oleum and hydrochloric acid:
 * 2 Se + 2 SO3 + 3 HCl → Se2Cl2 + SO2 + H2O + SO2(OH)Cl

The crude diselenium dichloride is removed via separatory funnel. Diselenium dichloride cannot be distilled without decomposition, even at reduced pressure.

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

Reactions
Diselenium dichloride is an electrophilic selenizing agent, and thus it reacts with simple alkenes to give bis(β-chloroalkyl)selenide and bis(chloroalkyl)selenium dichloride. It converts hydrazones 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.