Structural anomalies in formate chemisorption on Ni(100)

Abstract

Optimization of the geometric structure for a formate moiety (HCOO) chemisorbed in a symmetric bidentate configuration at a bridge site on a model Ni(001) surface reveals exceedingly long nickel–oxygen bonds. We report the results of generalized valence bond and configuration interaction calculations in which the formate oxygen atoms are 1.45 Å above the Ni (cluster) surface. This is well outside the ‘‘normal’’ range for nickel–oxygen bond lengths on surfaces or in bulk compounds. We find an adsorption enthalpy for the formate of 492 kJ/mol and a nickel–oxygen stretch frequency of 335 cm−1. The configuration found in these calculations for Ni(100) is very similar to that recently reported for formate chemisorption on a Cu(100) surface. The bonding is interpreted in terms of a charge-transfer chemisorption interaction that is offset by nonbonding repulsive interactions between adsorbate orbitals and the surface.