Apparently, this effect overcompensates the Pd-CO bond strengthening expected from the vertical ligand effect (interaction of surface Pd with Ag in the second layer), which would lead to a stronger Pd-CO bond. This case demonstrates nicely that the prediction of trends in adsorption energies can be difficult in complex situations like the above example, where several counteracting effects have to be considered. Nevertheless, in most situation it allows a simple estimate of trends in adsorption energies.