Density functional theory calculations of the transition-state structures and reaction barriers for the C-C coupling between monosubstituted eta(2)-olefins and eta(1)-vinyl for neutral [Pdl(PH3)(vinyl)(RCHCH2)] and cationic [Pd (H2PCH2PH2)(vinyl)(RCHCH2)](+) (R = OMe, Me, and CN) depend mostly on the regiochemistry and not on the starting position of the olefin substituent. The regiochemistry is thus implicit in the electronic structure of the precursor complex. A selectivity index, Omega, based on electrostatic and frontier orbital interactions gives a good correlation with experiment for vinylations or arylations. The model correctly predicts that the regiochemistry for R = OMe, Me, and CN is the same for both neutral and cationic Pd complexes while for R = CH2OH the regiochemistry reverses. The latter is confirmed by explicit calculations of the transition-state energies. Selectivity indices are computed for 13 substituents: CO2Me, CN, CF3, Ph, H, Me, CH2OH, CH2NMe2, 2-pyrolidone, CH2SiMe3, OAc, OMe, and F. Cationic conditions systematically give larger Q values and thus tend to favor coupling at the a carbon on the olefin. The Omega values are approximately additive and can be used to predict the regiochemistry for disubstituted olefins.