Electronic structure of oxo-bridge iron(III) dimers

Magnetic susceptibility results and extensive electronic, infrared, and Mi:issbauer spectral data are presented for enH_2[(FeHEDT A)_2O]· 6H_2O_3, Na_4[(FeEDTA)_2O]· 12H_2O, FeHEDTA· H_2O, and NaFeEDT A· 3H_2O (HEDTA = N-hydroxoethylethylenediaminetriacetate, EDTA = ethylenediaminetetraacetate, and enH_2^(2+) = cation). The magnetic and spectral data establish an electronic structural model for the -bridged dimers in which pairs of S = 5/2 Fe(III) ions interact antiferromagnetically, with J ≃ -95 cm^(-1). The -bridged dimers show marked intensity enhancement of the one-center Fe(III) ligand-field bands. There are several uv bands which are interpreted as arising from simultaneous electronic excitations of Fe(III) pairs. A high-spin ligand-field model modified by spin-spin interaction is judged to be considerably more appropriate the Dunitz-Orgel molecular orbital approach as a vehicle for describing oxo-bridged Fe(IIl) dimers.