An atom superposition and electron delocalization molecular orbital study shows the oxygen end of CO adsorbed on a cluster model of the Pt(l 11) surface is attracted by ir donation to empty d orbitals in Ti and Fe cations and an Fe ad-atom at an adjacent site. This attractive interaction causes tilting from normal and leads to decreases in the CO vibrational frequency of 3-400 cm™1. Adsorbed CO is not attracted toward the closed-shell d10 Zn2+ cation or toward the nearly closed-shell Pt ad-atom because the antibonding counterparts to the it donation orbitals are occupied in these cases. These results are related to the strong metal-support interaction literature by way of a brief review of that literature. The predicted tilting with CO frequency decrease supports Sachtler's conjecture to this effect, but it is suggested that it is still not certain that such CO bond weakening is a necessary or sufficient condition for Fischer-Tropsch selectivity.

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