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 C 2 H 4 and C 3 H 6 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 C 2 H 4 and C 3 H 6 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−π* back-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 z 2 orbital to the d yz orbital. Such redistribution apparently enhances the d−π* back-donation.