These considerations with respect to the geometric and ligand effects can qualitatively also been derived in a simple bond order concept . The situation depicted in Figure corresponds to an increase of the bond order of the considered surface metal atom, as either the number of the chemical bonds with the neighboring atoms or their strength is increased. The higher the bond order of an atom is, the less prone the atom is to make additional bonds to other species such as adsorbing atoms or molecules. Thus, a higher bond order of the respective surface atom leads to a lower bond strength to adsorbed species, which in the d-band model is reflected by a down-shift of the d-band center.