  We study the impact of electrode band structure on transport through
single-molecule junctions by measuring the conductance of pyridine-based
molecules using Ag and Au electrodes. Our experiments are carried out using the
scanning tunneling microscope based break-junction technique and are supported
by density functional theory based calculations. We find from both experiments
and calculations that the coupling of the dominant transport orbital to the
metal is stronger for Au-based junctions when compared with Ag-based junctions.
We attribute this difference to relativistic effects, which results in an
enhanced density of d-states at the Fermi energy for Au compared with Ag. We
further show that the alignment of the conducting orbital relative to the Fermi
level does not follow the work function difference between two metals and is
different for conjugated and saturated systems. We thus demonstrate that the
details of the molecular level alignment and electronic coupling in
metal-organic interfaces do not follow simple rules, but are rather the
consequence of subtle local interactions.
