Bonding of CO to metal surfaces: A new interpretation

Abstract

The interaction of CO with Na, Mg, and Al surfaces has been studied using the molecular-orbital cluster model. These metals are chosen since we wish to study the CO interaction with the metal valence $\mathrm{sp}$ electrons in the absence of $d$ electrons. Our conclusion is that there is substantial $\ensuremath{\sigma}$ repulsion and that the bonding arises primarily from the metal $\ensuremath{\pi}$ to CO $2{\ensuremath{\pi}}^{*}$ interaction and donation. This is based on the analysis of self-consistent-field calculations for linear $X\mathrm{CO}$ and two-layer ${X}_{4}(1,3)\mathrm{CO}$ clusters, where $X$ represents the metal atom. They are chosen to simulate normal adsorption at the on-top site with the C atom adjacent to the surface, a geometry known to be appropriate for certain transition-metal atoms. The potential curve for the interaction as a function of metal-C distance has an attractive or repulsive character directly related to the amount of adsorption-site $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ character in the bare (metal atoms only) cluster. A corresponding orbital analysis unambiguously shows that the free CO orbitals are essentially unchanged upon interaction with the metal; there is very little CO $5\ensuremath{\sigma}$ to metal donation. It also shows that the binding of CO to the metal is associated with a considerable change and donation of the metal $\ensuremath{\pi}$ electrons to CO $2{\ensuremath{\pi}}^{*}$, The $\ensuremath{\sigma}$ orbitals of primarily metal character hybridize and polarize away from CO in order to reduce the metal CO $5\ensuremath{\sigma}$ repulsion. This interpretation, which is different from the usual picture of the metal-CO bond, is supported by an extensive set of results. We also consider the effects of the interaction for the valence photoelectron peaks of chemisorbed CO and obtain a new interpretation for the significance of the shift of the $5\ensuremath{\sigma}$ ionization toward the $1\ensuremath{\pi}$. Predicted features of the CO ionization are compared to experimental photoemission spectra for CO/A1(111). Their agreement provides support for an adsorption geometry with CO normal to the surface.