The bonding of CO to metal surfaces

The atom and symmetry specific properties of x-ray emission spectroscopy ha ve been applied to the investigation of CO adsorbed on Ni(100) and Cu(100) surfaces. In comparison to ab initio electronic structure calculations, obt ained in density functional theory, we develop a consistent electronic stru cture model of CO adsorption on transition and noble metals and extend to a conceptual model of the surface chemical bond. A strong CO-substrate inter action is found, characterized by significant hybridization of the initial CO orbitals and the metal bands. In the pi system an allylic configuration is found as the result of orbital mixing between the CO 1 pi, 2 pi* and the metal d(pi)-band which is manifested experimentally in the observation of an oxygen lone-pair state. In the sigma system experimental evidence of equ ally strong orbital mixing has been found. Energetically, the adsorbate-sub strate complex is stabilized by the pi-interaction but is destabilized by t he sigma-interaction. Furthermore, the internal C-O bond carried by the pi- interaction is weakened upon adsorption, which is opposite for the internal C-O sigma bond that is strengthened. The equilibrium properties of CO adso rbed on these metals are found to be the direct result of the balance betwe en the sigma- and pi-interactions; both in terms of the total energy and th e local bond properties. (C) 2000 American Institute of Physics. [S0021-960 6(00)30702-4].