Selenium oxydichloride is the inorganic compound with the formula SeOCl<sub>2</sub>. 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 thionyl chloride SOCl<sub>2</sub>, being a pyramidal molecule.
Selenium oxydichloride can be prepared by several methods, and a common one involves the conversion of selenium dioxide to dichloroselenious acid followed by dehydration:
The original synthesis involved the redistribution reaction of selenium dioxide and selenium tetrachloride.
Pure selenium oxydichloride autoionizes to a dimer:
The SeOCl<sub>2</sub> is generally a labile Lewis acid and solutions of sulfur trioxide in SeOCl<sub>2</sub> likely form [SeOCl]<sup>+</sup>[SO<sub>3</sub>Cl]<sup>−</sup> the same way.
The compound hydrolyzes readily to form hydrogen chloride and selenium dioxide, and very few organic compounds dissolve in it without reaction. At elevated temperatures, it is a strong oxidizer, yielding a chloride, selenium dioxide, and diselenium dichloride.