Electronic Localization versus Delocalization Determined by the Binding of the Linker in an Isomer Pair

By using stoichiometric amounts of (C5H5)2FePF6, the isomeric neutral diamidate-bridged molecules, α- and
β-(DAniF)3Mo2(ArN(O)CC(O)NAr)Mo2(DAniF)3, with Ar = p-MeOC6H4, have been oxidized to give the PF6 salts of
the four cations α1+, α2+, β1+, and β2+. All four structures have been accurately determined and, together with
supporting evidence from near-IR, EPR, NMR and magnetic susceptibility measurements, it clearly establishes that
in the mixed-valent α+ species the unpaired electron is localized over only one of the Mo2 units while the α2+
cation behaves as a diradical having two Mo25+ units that are essentially uncoupled. However, the β+ species is
fully delocalized, in the time scale of the experiments, with the unpaired electron being equally shared by the two
Mo2 units. It displays a HOMO−1 → SOMO transition at 4700 cm-1 (Δν1/2 = 2300 cm-1). Because of strong
coupling, the β2+ species is diamagnetic.