A detailed density functional study was performed for the vinyl-vinyl reductive elimination reaction from bis-sigma-vinyl complexes [M(CH=CH2)(2)X-n]. It was shown that the activity of these complexes decreases in the following order: Pd-IV, Pd-II > Pt-IV, Pt-II, Rh-III > Ir-III, Ru-II, Os-II. The effects of different ligands X were studied for both platinum and palladium complexes, which showed that activation barriers for C-C bond formation reaction decrease in the following order: X = Cl > Br, NH3 > I > PH3. Steric effects induced either by the ligands X or by substituents on the vinyl group were also examined. In addition, the major factors responsible for stereoselectivity control on the final product formation stage and possible involvement of asymmetric coupling pathways are reported. In all cases DeltaE, DeltaH, DeltaG, and DeltaG(aq) energy surfaces were calculated and analyzed. The solvent effect calculation shows that in a polar medium halogen complexes may undergo a reductive elimination reaction almost as easily as compounds with phosphine ligands,