Structural Preferences for CO Bonding Modes in Transition Metals on Ag(110)

Pooja S Pathak, Robert Koniecny and Roald Hoffmann, Department of Physics, Brown University?

The orientation of the bond between CO and transition metals (Z=23 to Z=30) on a Ag(110) surface has been studied by molecular orbital calculations of the extended Huckel type. The system is modeled by first constructing a silver(110) surface from the bulk fcc crystal, upon which a single metal atom is chemisorbed. The C-O is then placed at the tip of the metal, changing the orientation of the C-O with respect to the metal. This arrangement makes up the unit cell which is extended in all three directions to model the composite system.
Upon studying the effect of the orientation of the metal-CO bond, it is observed that in early transition metals (Vanadium, Chromium, Manganese) the M-C-O bond is at 40 degrees from the axis perpendicular to the surface. In late transition metals (Cobalt, nickel , copper, zinc) the M-C-O are linear and perpendicular to the Ag(110) surface. Iron shows no preference for orientation. The change in trend while moving across the periodic table can be explained with the number of valence electrons and the metal electronegativity with respect to silver.