ClBEtC(Et):CRSiMe2Cl (I; R = Me, CH2:CMe) reacts with Na2Se to give the title compds. II (X = Se). I (R = Me) reacts with Na2Se2 to give II (X = Se, R = Me) and the six membered heterocycle (Me2SiSe)3. II (X = Se, R = Me) was also formed along with cyclophosphines by the reaction of II ( X = PhP, R = Me) with Se. The same reaction with S gave II ( X = S, R = Me) along with cyclophosphines. II (X = Se, R = Me) reacts with MeOH to give II ( X = OMe) whereas with acetylacetone, II (X = O) along with cyclic borate complex C5H7O2B(Et)SeSiMe2CMe:CHEt was obtained. Some 1:1 adducts were prepd. by the reaction of II (X = Se; R = Me, CH2: CMe) with AlCl3, pyridine, and Me3P. Photochem. or thermal reaction of II (X = Se; R = Me, CH2:CMe) with Fe(CO)5, Ru3(CO)12, (cyclododecatriene)nickel, [(h5-C5H5)Co(C2H4)2] gave complexes of the type III or IV. The crystal structure of III (R = Me) was detd.