  We study the impact of the valence and the geometry on the electronic
structure and transport properties of different transition metal-benzene
sandwich molecules bridging the tips of a Cu nanocontact. Our
density-functional calculations show that the electronic transport properties
of the molecules depend strongly on the molecular geometry which can be
controlled by the nanocontact tips. Depending on the valence of the transition
metal center certain molecules can be tuned in and out of half-metallic
behaviour facilitating potential spintronics applications. We also discuss our
results in the framework of an Anderson impurity model, indicating cases where
the inclusion of local correlations alters the ground state qualitatively. For
Co and V centered molecules we find indications of an orbital Kondo effect.
