Selenium monochloride

Dielenium dichloride has the connectivity Cl−Se−Se−Cl. 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 angle Se, Se, Cl is 104° and the dihedral angle between the Cl^a−Se−Se and Se−Se−Cl^b planes is 87°.

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

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.