Ab initio and density functional theoretical investigations into the nature of the reaction of acetylene with noncluster organocuprate reagents, MeCu, Me2Cu-, and lithium organocuprate cluster reagents, Me2CuLi, Me2CuLi . LiCl, and (Me2CuLi)(2) were carried out, and the function of the mixed metal cluster was probed. Intermediates transition structures (TSs) as well as the structures on the intrinsic reaction coordinate in the C-C bond forming stage of the reaction of lithium cuprate clusters have been calculated with the ab initio method (MP2) and density functional method (B3LYP) using all-electron basis sets for copper. The addition reaction of the simple organocopper reagent, MeCu, can be viewed as a simple four-centered addition reaction consisting of nucleophilic addition of an anionic methyl group, while the addition of the cuprate reagent, Me2Cu-, involves transfer of negative charge to the acetylene via the copper atom. In the cluster reaction of Me2CuLi . LiCl, the lithium atom in the cluster stabilizes the developing negative charge on the acetylene moiety and assists the electron flow from the copper atom. Reductive elimination of the transient Cu(III) species initially gives a 1-propenyllithium-like structure intermediate (nonstationary point), which then undergoes intramolecular transmetalation to give the final product, 1-propenylcopper, Essentially the same mechanism operates also with Me2CuLi and (Me2CuLi)(2), indicating that the Li-Me-Cu-Me moiety incorporated in the mixed organocuprate cluster is essential for the reaction. Experiments showed that the strong solvation of the lithium atom with a crown ether, which sequesters the lithium cation from the cluster, strongly decelerates the carbocupration reaction. Thus, theory and experiments revealed the cooperative function of lithium and copper atoms in the cuprate reactions.