Abstract: Crystalline methylselenium trichloride was prepared by the chlorination of methylene chloride solutions of dimethyl diselenide. Interestingly, molecular weight data suggest methylselenium trichloride exists mainly as a dimeric species in methylene chloride. Infrared and proton magnetic resonance spectra are consistent with this view. Unlike the di- and monochlorides methylselenium trichloride did not form an isolable adduct with boron ...