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, alpha- and beta-(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 alpha1+, alpha2+, beta1+, and beta2+. 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 alpha+ species the unpaired electron is localized over only one of the Mo2 units while the alpha2+ cation behaves as a diradical having two Mo25+ units that are essentially uncoupled. However, the beta+ 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 (Deltanu1/2 = 2300 cm-1). Because of strong coupling, the beta2+ species is diamagnetic.