Bis(2-chloroethyl)selenide
Bis(2-chloroethyl)selenide is the organoselenium compound with the formula Se(CH2CH2Cl)2. As a haloalkyl derivative of selenium, it is an analogue of bis(2-chloroethyl)sulfide, the prototypical sulfur mustard used in chemical warfare.
Source: Wikipedia — Bis(2-chloroethyl)selenide (CC BY-SA 4.0)