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 electronic structure of chemisorbed ethylene and benzene on the Cu(110) surface to investigate the suitability of the donation/back-donation bonding model given by Dewar 1 and Chatt and Duncanson 2 (DCD) for the interaction of unsaturated hydrocarbons with metal surfaces. We give an experimental verification of the DCD model and find donation/back-donation to be twice as large for ethylene as for benzene. In particular, the degree of σ−π mixing (rehybridization) is found to correspond to the amount of donation/back-donation, which is put in relation to the aromatic and nonaromatic π characters of benzene and ethylene, respectively.