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

An ab initio molecular orbital study using the effective core potentials (E CP) is performed to determine the anion and cation effects on the adsorptio n of C2H4 and C3H6 on CuX and AgX (X = F, Cl, Br, I). Compared with all-ele ctron calculations, the ab initio ECPs require only a fraction of the compu tational resources with accuracy that approaches that of the all-electron c alculations. 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 ag reement with the experimental results. In addition, the theoretical metal-o lefin bond energies are in fair agreement with the experimental data. A det ailed analysis of the electronic distribution is also performed using natur al bond orbital (NBO) theory. The NBO results show that the d-pi* back-dona tion is about twice that of the sigma donation for the Cu+-olefin bonds, wh ereas the two donations are of the same order for the Ag+-olefin bonds. Hen ce, the Cu-C bonds contain more metal d than metal s character. For both me tals, there is considerable electron redistribution upon olefin bonding, fr om the d(z2) orbital to the d(yz) orbital. Such-redistribution apparently e nhances the d-pi* back-donation.