An ab initio molecular orbital study using the effective core potentials (ECP) is performed to determine the anion and cation effects on the adsorption of C2H4 and C3H6 on CuX and AgX (X = F, Cl, Br, I). Compared with all-electron calculations, the ab initio ECPs require only a fraction of the computational resources with accuracy that approaches that of the all-electron calculations. The following trends of anion and cation effects were obtained for the adsorption of C2H4 and C3H6 on the metal halides: F- > Cl- > Br- > I- for anions, and Cu+ > Ag+ for cations. These trends are in excellent agreement with the experimental results. In addition, the theoretical metal-olefin bond energies are in fair agreement with the experimental data. A detailed analysis of the electronic distribution is also performed using natural bond orbital (NBO) theory. The NBO results show that the d-pi* back-donation is about twice that of the sigma donation for the Cu+-olefin bonds, whereas the two donations are of the same order for the Ag+-olefin bonds. Hence, the Cu-C bonds contain more metal d than metal s character. For both metals, there is considerable electron redistribution upon olefin bonding, from the d(z2) orbital to the d(yz) orbital. Such-redistribution apparently enhances the d-pi* back-donation.