Selenium monochloride or diselenium dichloride is an inorganic compound with the formula . 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 , , chlorine, and elemental selenium. Diselenium dichloride is mainly used as a reagent for the synthesis of Se-containing compounds.
Dielenium dichloride has the connectivity . 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 and planes is 87°.[1]
Early routes to diselenium dichloride involved chlorination of elemental selenium.[2] An improved method involves the reaction of a mixture of selenium, selenium dioxide, and hydrochloric acid:[3]
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:[3]
The crude diselenium dichloride is removed via separatory funnel. Diselenium dichloride cannot be distilled without decomposition, even at reduced pressure.[3]
In acetonitrile solutions, it exists in equilibrium with and .[4] Selenium dichloride degrades to diselenium dichloride after a few minutes at room temperature:[5]
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.[6]