The addition of ethylene to solutions of 4-tolueneselenenyl chloride (ArSeCl) gives initially the selenide dichloride 5. Only after more than 20% of the theoretical amount of ethylene has been added does the normal adduct appear. This anomaly is caused by the reversible reaction of the normal adduct 4 with ArSeCl, giving diselenide and 5. The episelenurane 1 reported as a product of the reaction of ArSeCl with ethylene appears to be actually the ...