Spin S = 1 in six-coordinated iron(II): Low-temperature magnetism, Mössbauer effect, and electronic structure

Magnetic susceptibilities down to 1.0°K and 57Fe Mössbauer spectra between 298 and 4.2°K are reported on Fe(phen)2ox· 5H2 O(I), Fe(phen)2mal· 7H2 O(II), and Fe(phen)2F2 · 4H2 O(III) (phen = 1,10-phenanthroline, ox = oxalate, mal = malonate). Between 300 and ∼ 15°K, the effective magnetic moments are almost independent of temperature at μeff = 3.76, 3.87, and 5.20 μB; below 15°K the moments start to decrease and, at 1.0° K and 8.60 kG, they assume values of 2.65, 2.64, and 3.71 μB, for I, II, and III, serially. A slight field dependence due to saturation effects is encountered in the cryogenic range. The results are consistent with a spin-mixed 3A2(ξ2η2εζ) ground state characterized by a zero-field splitting of |D|= 2.0–3.0 cm−1 and g values significantly higher than 2.0. The large effective g is indicative of mixing with the higher 5T2 multiplet. From the Mössbauer spectra, the quadrupole splittings were obtained as ΔEQ = 0.24, 0.26, and 0.25 mm sec−1, the isomer shifts as δIS= +0.29, +0.30, and +0.30 mm sec−1, for I, II, and III, serially, and both are temperature independent. In reasonable agreement, the quadrupole splitting of a 3A2 state is calculated as zero. The discrepancy with iron(II) phthalocyanine is resolved in that, for a 3B2 ground state, ΔEQ is calculated as consistent with experiment (ΔEQ = 2.70 mm sec−1).