). Since the charge on the middle layer adsorption site (Ru-5a) is essentially identical with that on the top-layer terrace site (Ru-1c), both for the fully oxidized surface and also in the presence of an O-vacancy (Table ), this larger adsorption energy cannot be simply explained by differences in the charge of the Ru adsorption sites before CO adsorption. We tentatively attribute this stronger bond between the Ru-5a atom and the adsorbed CO to a lower coordination of this Ru surface atom, due to larger bond lengths to some of the neighboring Ru atoms. For the Ru-5a atom, only four Ru neighbors show bond distances of up to 2.70 Å, the Ru-Ru distance in bulk Ru, while for the Ru edge atoms 7a and 8a, six neighboring Ru atoms can be found within this distance. This may shift the d-band center without significant changes in the total charge. Such effects will be discussed in more detail later in this section.