Observation and interpretation of 157.5 T internal magnetic field in Fe[C(SiMe₃)₃]₂ coordination compound

Abstract

The compound Fe[C(SiMe₃)₃]₂ has been prepared and investigated by the means of powder X-ray diffractometry and ⁵⁷Fe Mössbauer spectroscopy. The compound’s unique geometry, in which iron is linearly coordinated by the two C(SiMe₃)₃ ligands, results in a unusual electronic structure of iron, which is visualized as an extreme high hyperfine magnetic field of 157.5(8) T as sensed by the ⁵⁷Fe nucleus at T = 20 K. In order to obtain information on the electronic structure of iron and on the bonds to the ligands, DFT (density functional theory) calculations were carried out on Fe[C(SiMe₃)₃]₂. The high-spin state of iron was found to be energetically favored: an Fe(II) electron configuration of 3d^5.83 4s^0.72 is predicted, where the 4s electron density is only slightly polarized, and most of the unpaired electrons have 3d character. By assuming a linear crystal field, and associated 3d level scheme as a starting point, it is suggested that the extreme high hyperfine magnetic field, observed along with an apparently negative quadrupole splitting, is perpendicular to the C–Fe(II)–C bond axis, and can be decomposed mainly into contact (B _c ≈ 44 T), dipolar (B _d ≈ 14 T), and orbital (B _L ≈ 99 T) hyperfine magnetic field contributions.