Anion and cation effects on olefin adsorption on silver and copper halides: Ab initio effective core potential study of π-complexation

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-πback-donation is about twice that of the σ 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-πback-donation.