Direct experimental measurement of donation/back-donation in unsaturated hydrocarbon bonding to metals

Using resonant and off-resonant X-ray emission spectroscopy, in combination with ground-state density functional calculations, we examine the electron ic structure of chemisarbed ethylene and benzene on thr Cu(110) surface to investigate the suitability of the donation/back-donation bonding model giv en by Dewar(1) and Chart and Duncanson(2) (DCD) for the interaction of unsa turated hydrocarbons with metal surfaces. We give an experimental verificat ion of the DCD model and find donation/back-donation to be twice as large f or ethylene as for benzene. In particular, the degree of sigma-pi mixing (r ehybridization) is found to correspond to the amount of donation/back-donat ion, which is put in relation to the aromatic and nonaromatic pi characters of benzene and ethylene, respectively.