Single-molecule circuits with group 8 metallocenesare formed without additional linker groups in scanning tunnelingmicroscope-based break junction (STMBJ) measurements atcryogenic and room-temperature conditions with gold (Au)electrodes. We investigate the nature of this direct gold-pi bindingmotif and its effect on molecular conductance and persistencecharacteristics during junction evolution. The measurementtechnique under cryogenic conditions tracks molecular plateausthrough the full cycle of extension and compression. Analysisreveals that junction persistence when the metal electrodes arepushed together correlates with whether electrodes are locallysharp or blunt, suggesting distinct scenarios for metallocenejunction formation and evolution. The top and bottom surfaces of the"barrel"-shaped metallocenes present the electron-rich pi system of cyclopentadienyl rings, which interacts with the gold electrodes in two distinct ways. An undercoordinated gold atom on asharp tip forms a donor-acceptor bond to a specific carbon atom in the ring. However, a small,flat patch on a dull tip can bind morestrongly to the ring as a whole through van der Waals interactions. Density functional theory (DFT)-based calculations of modelelectrode structures provide an atomic-scale picture of these scenarios, demonstrating the role of these bonding motifs duringjunction evolution and showing that the conductance is relatively independent of tip atomic-scale structure. The nonspecificinteraction of the cyclopentadienyl rings with the electrodes enables extended conductance plateaus, a mechanism distinct from thatidentified for the more commonly studied, rod-shaped organic molecular wires.
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