SCF-Xα scattered wave cluster study of molecularly adsorbed NO on Ni

SCF-Xα SW calculations have been performed on model clusters representing molecularly adsorbed NO on different sites of Ni surfaces: on-top and fourfold hollow positions on Ni(100) and a threefold site on Ni(111). The chemisorption bond is found to be formed through adsorbate-to-metal donation mainly via 4σ̃gs and 1π̃gp orbitals and back donation into the 2π^* orbital of NO. The molecular 2π^* orbital interacts with a manifold of substrate levels leading to a "broadening" of this level. A strong mixing of the 2π^* orbital both to the 1 \ ̃gp cluster orbital and to the band like 2π manifold leads to a weakening of the intramolecular bond of the adsorbate, especially for hollow site adsorption. The chemisorptive bond strength is found to increase with the metal coordination of the adsorption site leading to a definite preference of hollow positions over on-top positions.