Finally, we would like to get back to the case of early transition metals with a less than half-filled dband. As already mentioned, in this case the charge conservation argument would lead to an opposite trend in the d-band shift as depicted in Figure . Increasing the coordination of the metal surface atoms or applying compressive strain would lead to an up-shift of the d-band and thus to a strengthening of an adsorption bond, opposite to the trend in late transition metals. This trend has indeed been observed in some early transition metals such as Sc, V and Ti, and has been demonstrated for H adsorption energies . Hence, while the d-band model still results in a correct prediction of the trend in adsorption energies, the correlation with the bond-order concept does not work any more, as the latter would predict a weakening of the metal-adsorbate bond for higher coordination of the metals surface atoms. Nevertheless, considering that most metal or bimetal catalysts involve late dband metals, the description of trends in adsorption energy in terms of the constant bond order principle is of great practical use.