of a given adsorption site / ensemble. These variations are induced, e.g., by the presence of larger or smaller surface atoms outside the adsorption ensemble 6 Trends in the relative adsorption strength of a given adsorption site or adsorption ensemble, which on a more quantitative scale can be derived from the shifts of the d-band center (d-band model), can be estimated rather well for the dominant late transition metals with a more than half filled d-band in a simple chemical picture, via the principle of constant bond order. This says that depending on the number of bonds from the adsorbing surface atom or adsorption site to neighboring atoms (structural effect) and on their bond strength (electronic effects), the bond to an additional atom/molecule, here the adsorbed species, will be lower or stronger to maintain a constant bond order. For early transition metals with less than half filled d-band, this approach does not work.