Electronic localization versus delocalization determined by the binding of the linker in an isomer pair

By using stoichiometric amounts of (C5H5)(2)FePF6, the isomeric neutral diamidate-bridged molecules, alpha- and beta-(DAniF)(3)Mo-2(ArN(O)CC(O)NAr)Mo-2(DAniF)(3), with Ar = p-MeOC6H4, have been oxidized to give the PF6 salts of the four cations alpha(1+), alpha(2+), beta(1+), and beta(2+). 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 Mo-2 units while the alpha(2+) cation behaves as a diradical having two Mo-2(5+) 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 Mo-2 units. It displays a HOMO-1 -> SOMO transition at 4700 cm(-1) (Delta nu(1/2) = 2300 cm(-1)). Because of strong coupling, the beta(2+) species is diamagnetic.