and HEAL suggested that the higher organochalcogen fluorides were unstable.1 Sheppard however has demonstrated the stability of organosulphur tri-and penta-fluorides.2 In a further study of methylselenium halides,s we have found more evidence for the stability of organochalcogen fluorides. We have synthesized the first reported example of a diorganoselenium difluoride, dimethylselenium difluoride, Me,SeF2. This compound was prepared from the reaction of dimethylselenide with silver( 11) fluoride in 1, 1,2-trichloro-l,Z,Z-trifluoroethane. Satisfactory C, H, Se, and F analyses were obtained.Although previously reported organoselenium dihalides have been found to be solids a t ambient temperature, Me,SeF, is a liquid freezing a t 11.2-1 1.4" which distils slowly under reduced pressure (0.01 mm.) a t 25". When exposed to moist air, MeSeF, rapidly forms a white solid which reacts slowly with glass. The liquid possesses moderate thermal stability, but decomposes above 120". Usually, alkylselenium halides are less thermally stable than the corresponding aryl derivatives. Thus, diarylselenium difluorides, when they are synthesized, should exhibit even better thermal stability.The lH n.m.r. spectrum of pure Me2SeF2 a t 100MHz and 30°, showed 8-3.13 (t, JHF 9.0 c./sec.) with tetramethylsilane as internal reference. The triplet starts to collapse a t about 50". These data indicate that rapid intermolecular fluorine exchange does not occur at ambient temperature. In contrast, 19F n.m.r. data for SeF, in propane showed that fluorine exchange could not be completely eliminated at -200°.4The structure of Me,SeF, is probably trigonal bipyramidal as shown. This is consistent with the n.m.r. data which indicate the presence of two equivalent fluorines, and with X-ray data on diorganoselenium dichlorides and dibromides.6