The interplay between the electronic and the geometric structure of adsorbates is of fundamental importance for the understanding of many surface phenomena. Using x-ray emission spectroscopy and ab initio cluster calculations, this issue has been investigated in unprecedented detail for CO adsorption in different adsorption sites. The investigation establishes $\pi$ bonding and $\sigma$ repulsion, both increasing with the number of coordinated metal atoms. The two contributions partly compensate each other, leading to only small differences in adsorption energies for the different adsorption sites despite very large variations in the electronic structure.

• Received 11 October 1999

DOI:https://doi.org/10.1103/PhysRevLett.85.3309