•Maarten Wegewijs1, Christian Romeike1, Wolfgang
Wenzel2 und Herbert Schoeller1
1Institut für Theoretische Physik - Lehrstuhl A , RWTH Aachen , 52056 Aachen , Germany
2Forschungszentrum Karlsruhe , Institut für Nanotechnologie , 76021 Karlsruhe , Germany
We theoretically consider electron tunneling through a transition-metal complex coupled to metallic electrodes. A single Co2+/3+ ion is fixed between a left and right terpyridine ligand, each of which is in turn coupled to a linker molecule with a functionalized end group to connect to one of the electrodes. It was found experimentally  that by varying the type of linker molecules, a strong Kondo effect in the current appears at low temperatures in Coulomb blocked transport regimes due to a stronger coupling to the electrodes.
We have investigated the conjecture that the tunneling is dominated by single atom-like d-orbitals of the ion by electronic structure calculations. We show that microscopically the ligands of this particular complex function as a tunnel barrier due to the different character (symmetry) of the highest ligand (p) and two Co2+/3+-ion orbitals (s). The role of the linker molecules connecting the ligands to the electrodes is therefore crucial. We discuss the simplest phenomenological tunneling model consistent with our findings (molecular states, addition energy, tunneling matrix elements).
 J. Park et al, Nature 417, 722 (2002)