Strongly Coupled, Oxo-Bridged Iron (III) Complexes Magnetically Perturbed Mössbauer Spectra

The electric field gradient tensor is used to study structure, spin state, and bonding in some representative binuclear iron (III) systems. Mössbauer spectra of strong $\text{(J\hspace{0.17em}≈\hspace{0.17em}−\hspace{0.17em}100}{\mathrm{cm}}^{\text{−1}})$ exchange coupled intramolecular antiferromagnets have been determined in zero applied field and also longitudinal fields up to 26 kG. The systems studied, [Fe(salen)]₂O (salen‐N, N′‐ethylenebissalicydeneiminate), [Fe(phen)₂Cl]₂‐OCl₂·5H₂O (phen‐1, 10‐phenanthroline), and [FeB(H₂O)]₂O(ClO₄)₄ {B=the macrocyclic ligant 2, 13‐dimethyl‐3, 6, 9–12, 18‐pentaaza‐bicyclo[12.3.1]octadeca‐1(18), 2, 12, 14, 16‐pentaene)} contain five‐, six‐, and seven‐coordinate iron (III), respectively, and show approximate axial symmetry. Signs for the principal component of the electric field gradient tensor are discussed in terms of point charge geometry and $\sigma$ bonding considerations. On this basis, a cis arrangement of chlorine and oxygen is proposed for [Fe(phen)₂Cl]₂‐OCl₂·5H₂O, and the nature of the large quadrupole splitting is discussed. The spin state of the individual iron (III) ions is also considered and is indicated to be $\frac{5}{2}$ . The absence of relaxation broadening and internal field augmentation at 4.2°K are correlated with total spin zero ground states for the binuclear complexes.