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 ${\mathrm{Fe(phen)}}_2\mathrm{ox}\text{· 5}{\mathrm{H}}_2\hspace{0.17em}\mathrm{O(I)},{\mathrm{Fe(phen)}}_2\mathrm{mal}\text{· 7}{\mathrm{H}}_2\hspace{0.17em}\mathrm{O(II)}$ , and ${\mathrm{Fe(phen)}}_2{\mathrm{F}}_2\text{· 4}{\mathrm{H}}_2\hspace{0.17em}\mathrm{O(III)}$ (phen = 1,10‐phenanthroline, ox = oxalate, mal = malonate). Between 300 and $\text{∼ 15°}\mathrm{K}$ , the effective magnetic moments are almost independent of temperature at μ_eff = 3.76, 3.87, and 5.20 μ_B; 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 μ_B, 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 $\text{|D|= 2.0–3.0\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ 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 ΔE_Q = 0.24, 0.26, and 0.25 mm sec⁻¹, the isomer shifts as ${\delta }^{\mathrm{IS}}\text{= +0.29, +0.30}$ , and $\text{+0.30\hspace{0.17em}}{\mathrm{mm\; sec}}^{\text{−1}}$ , 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, ΔE_Q is calculated as consistent with experiment (ΔE_Q = 2.70 mm sec⁻¹).