Spectroscopic Studies of Spin State and Electron Delocalization in a Binuclear Mixed-Valence Fe(II/III) Complex

A brief historical survey is given of the “doubleexchange” interaction, by which the spins of the paramagnetic centers in mixed-valence systems are ferromagnetically coupled through quantum-mechanical electron delocalization. The spectroscopic evidences, obtained by UV-visible spectrometry, EPR, and Mossbauer spectroscopy, for electron delocalization and ferromagnetic spin coupling in the binuclear triply ligand-bridged Fe(II/III) molecular complex [L2Fe2(μ-OH)3] (C104)2 • 2CH3OH-2H2O, with L = N, N’,N”-trimethyl 1, 4, 7-triazacyclononane are summarized. Values for the double-exchange parameter B (±1300 cm-1) and the Heisenberg exchange-coupling constant J (-180 cm-1 in JS1 S2) in the dimeric iron unit are inferred from UV-visible and magnetic-susceptibility data. The electric and magnetic hyperfine parameters, deduced from the Mossbauer spectra, correlate with an “odd” electron that is delocalized over two do orbitals, oriented along the Fe-Fe axis. A condition for electron delocalization, obtained with the PKS model, is briefly discussed.