Preparation of (Perfluoroalkyl)halogeno‐1,3‐diselenetanes from the Corresponding Selenocarbonyl Fluorides and Reactions with Boron Trichloride or Arsenic Pentafluoride1)Reactions of Hg(SeRt)2(Rt= C2F5, C3F7, CF3) with (C2H5)2All or All3in octamethylcyclotetrasiloxane produce the unstable perfluoroalkaneselenocarbonyl fluorides1[R1= CF3(1 a), C2F5(1 b), CF3Se (1 c)]. These compounds are very reactive and polymerize to rubberlike products. On heating the polymers decompose almost quantitatively to the monomers or dimers. In CFCl3solution1dimerizes at 20°C in sunlight to the correspondingcis/trans‐1,3‐diselenetanes2. The structure of2 bis determined by single crystal X‐ray diffraction. Different selenocarbonyl derivatives add to unsymmetrically substituted 1,3‐diselenetanes3.1 aand1 breact with cyclopentadiene to form the [4 + 2] cycloaddition products4aand4c. Halogen exchange reactions take place between2a—dand BCl3. Thecis‐isomers react much faster than thetrans‐isomers to give a mixture ofcis‐,trans‐forms of5. When2c,dis treated with BCl3it is possible to isolate and characterize the puretrans‐isomer2d, which is separated by preparative gas chromatography from5c,d. Fluorine abstraction with formation of 1,3‐diselenetan‐2‐ylium ions (6a—f) are accomplished by