Abstract Regio-and stereoselective electrophilic addition reactions of SeCl 2, SeBr 2, SeCl 4, SeBr 4 to methyldiethynylsilane lead to predominant formation of the Z-isomers of 3, 6- dihalo-4-methyl-1, 4-selenasilafulvenes, disiloxanes on their bases, as well as disiloxanes formed upon the reaction with methyldiethynylsilane. TeCl 4 reacts with methyldiethynylsilane with predominant formation of the E-isomer of 1, 1, 3, 6-tetrachloro- ...