  Density functional theory within the generalized-gradient approximation is
used to study the adsorption of the isocyanides CNH and CNCH3 on the gold (111)
surface at several coverages. It is found that these molecules are highly
selective in their adsorption site preference. Adsorption is possible only at
the top site, although binding is rather weak and dipole-dipole repulsion
prevents binding at coverages of 1/3 ML and higher. At all other high symmetry
sites considered (hcp, fcc, bridge), the isonitriles are not bound. To reveal
the main mechanisms of bonding, the isonitriles are compared to CO and ammonia
using Au-X radicals as model systems. The systematic trends are understood in
terms of $\sigma$ donation from the ligand lone pair and $\pi^*$ back donation.
Finally, CNH is found to be more strongly bound to an undercoordinated gold
atom (adatom) on the surface.
