Several organosilylcuprates have been prepared and their reactions with acetylenes examined. It has been shown that LiCu[Si(SiMe 3) 3] 2 adds to acetylenes in only one manner, i.e. with cis addition and the Si(SiMe 3) 3 group in the least hindered cite as shown in the equation below. Similarly the sterically hindered LiCu(SiMePh 2) 2 LiCu[Si(SiMe 3) 2] 2·LiI + RCCH → H +,H 2O → trans-RHCCHSi(SiMe 3) 3 adds only in this fashion. If the less sterically hindered lithium compound, LiSiMe 2Ph, is used to prepare the copper reagent, then, when the Li/Cu ratio is reduced to 1 1 a mixture of products, A and B (SiR 3 = SiMe 2Ph; R′ = n=Bu) is obtained in a ratio of 2 1 , but only one product, A (SiR 3 = SiMe 2Ph; R′ = n-Bu, t-Bu, Ph), is obtained when the ratio of Li/Cu is 2 1 . Only A is obtained when either diphenylmethylsilyllithium or tris(trimethylsilyl)silyllithium (SiR 3 = SiMePh 2, Si(SiMe 3) 3; R′ = n-Bu, t-Bu, Ph) is used regardless of the Li to Cu ratio. ▪