The complexes [{Fe(η5-C5Me5)} 2(μ2,η12-phenanthrene)]n+ have been synthesized for n=0-2; the green 37-electron mixed-valence Fe IFeII monocation is delocalized on the Mössbauer time-scale at 4 K and the bireduced neutral complex has a triplet Fe IFeI state for phenanthrene in contrast with the singlet FeIIFeII state for the dihydrophenanthrene complex; interconversion between the phenanthrene and dihydrophenanthrene series by contact with O2and H2O makes this system a switchable molecular-electronic device.