A new series of diiron polyaromatic complexes, [(FeIICp*)2(μ2-η12-polyaromatic)]2+(PF6-)2 (Cp* =η5-C5Me5; polyaromatic = dihydrophenanthrene, fluorene, diphenylmethane, fluorene, phenanthrene, triphenylmethane, pyrene, [2.2]paracyclophane), has been synthesized by reaction of [FeCp*(CO)2Br] with Al2Cl6 and Al2Cl3Me3, avoiding hydrogenation of the polyaromatic ligand. One-electron and two-electron reductions have been achieved to investigate the electronic communication between the two iron atoms in the 37-electron (37e) mixed-valence complexes and in the 38e complexes. The Cp* (Cp* =μ5-C5Me5) ligand stabilizes the 37e mixed-valence complexes which were synthesized by comproportionation between the 36e and 38e complexes. Mössbauer spectra show that these complexes are average valence on the Mössbauer time scale. The 38e complexes are thermally stable up to -20 °C and were studied by 1H NMR, ESR, and Mössbauer spectroscopy for phenanthrene, pyrene, and triphenylene. The absence of 1H NMR spectra in the diamagnetic region and the three g values around 2 in ESR are characteristic of 19e FeI complexes and indicate that these complexes do not undergo intramolecular chemical coupling (formation of the exocyclic double bond), in contrast to the dihydrophenanthene and cyclophane complexes, which are shown by 1H NMR to be diamagnetic. The compared Mössbauer parameters of the 36e FeIIFeII, 37e FeIFeII, and 38e FeIFeI polyaromatic complexes mentioned above are very similar, indicating that the 37th and 38th electrons are in mainly polyaromatic-based orbitals. The Mössbauer IS and QS values of the 38e complexes, however, are slightly higher than those of the 36e and 37e series, consistent with a slightly higher metal contribution. The cyclic voltammograms of the 36e polyaromatic complexes show a redox series of five oxidation states, contrasting with those of dihydrophenanthrene and [2.2]paracyclophane, which give only two waves because of intramolecular chemical coupling at the second reduction stage. Among the four one-electron waves, only the last one is chemically and electrochemically irreversible, showing the structural rearrangement in the course of the fourth reduction. SCF MS-Xα calculations have been performed on the diiron pyrene species and were compared to similar calculations carried out on the related diphenyl system. The reduction of the 36e diphenyl complex leads to the occupation of its buLUMO, which is significantly π-bonding on the exocyclic bond, thus inducing chemical coupling. In the case of the 36e pyrene complex, the corresponding bu vacant orbital is not the LUMO and has a poorer π-bonding character on the C-C bond susceptible to be involved in the chemical coupling. Therefore, this bu level is not involved upon reduction. Instead, the au LUMO and the next ag level are successively populated by the incoming electrons, the latter being more metal-centered than the former. © 1995, American Chemical Society. All rights reserved.