Examining the Factors Controlling Transition Metal Surface Chemistry in terms of Local Bonding Interactions and Metal Bond Conjugation

Although a comprehension of adsorption to transition metal surfaces is crucial to understanding the principles that control catalytic chemistry on these surfaces, the nature of the metal-adsorbate bond is not understood to the same extent as bonding in organic molecules and organometallic complexes. This is due to the typical characterization of the metal-metal bonding within the transition metal surface as being delocalized over the entire crystal. In this work, a formalism is introduced that describes metal-metal bonding in terms of local fractional bonds that interact with each other through conjugation.
The resulting model is first used to describe the bonding in bulk 5d transition metals, using sd^n hybrid orbitals to form these bonds. It is found that conjugative interactions between vicinal metal-metal bonds account for a large portion of the cohesive energy, indicating that a strictly local representation of metal-metal bonding is insufficient. The trends in cohesive energy and surface energy of the (111) surface calculated from this model follow the same trend as the values calculated using density functional theory, indicating that the longer range delocalization excluded from this model is not required to describe the main features of the variation of bond strength among the different metals examined.
Application of this model to the chemisorption of atomic hydrogen also yields values of the adsorption energy that follow the same trend as the values calculated by density functional theory. The low adsorption energy on Au compared to the other 5d metals is found to be due to two effects. The first of these is due to the fact that the entire d shell is filled in the ground state of the Au atom so that the relevant d orbital cannot participate in forming a metal-hydrogen bond unless part of an electron is first promoted from this orbital into the higher energy s orbital. The second effect is due to the bond saturation of the Au atom by formation of a single bond to hydrogen so that it can no longer participate in conjugative interactions with neighboring metal atoms in the surface. These insights are applied qualitatively to a variety of other adsorbates, explaining a wide range of chemisorption behavior on these surfaces.