Role of Orbital Degeneracy in the Single Molecule Magnet Behavior of a Mononuclear High-Spin Fe(II) Complex

Abstract

To explain the single-molecule magnet behavior of the mononuclear complex [(tpaMes)Fe](-) we have developed a model that takes into account the trigonal ligand field splitting of the atomic (5)D term of the Fe(II) ion, and the spin-orbital splitting and mixing of the ligand field terms. The ground ligand field term is shown to be the orbital doublet (5)E possessing an unquenched orbital angular momentum. We demonstrate that the splitting of this term cannot be described by the conventional zero-field splitting Hamiltonian proving thus the irrelevance of the spin-Hamiltonian formalism in the present case. The first-order orbital angular momentum is shown to lead to the strong magnetic anisotropy with the trigonal axis being the easy axis of the magnetization.

Mononuclear FeII Single-Molecule Magnets: A Theoretical Approach

“…16 However, such degeneracy must strictly appear in nondistorted complexes, which is not the case for the studied systems due to the JahnÀTeller effect. The analysis of the CASSCF-RASSI function indicates the presence around 20% of the orbital component for the studied nonsymmetric Fe II complexes.…”

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…This behavior was attributed to the thermal depopulation of the m s = 0 ± 1 levels promoting ground-state quantum tunneling. Another model that considers the trigonal ligand field and the SOC was proposed by Palii et al to explain the magnetic behavior of 28 [49]. In this case, the trigonal crystal field splits the atomic 5 D term into two orbital doublets (states with non-zero orbital momenta) and one orbital singlet.…”

“…[52]. For the d 6 configuration (i.e., high-spin Fe II or high-spin Co III ), the degenerate d xy and d yz orbitals are partially filled (occupied with three electrons), and thus the orthogonal rotational transformation along the z axis between these two degenerate orbitals could cause significant first-order SOC, thereby resulting in a large D parameter [86,95,96]. However, Jahn-Teller distortion is inevitable for high-spin d 6 metal ions with ideal TBP geometry, which reduces the first-order SOC due to degeneracy breaking.…”

Terminological confusions and problems at the interface between the crystal field Hamiltonians and the zero-field splitting Hamiltonians—Survey of the CF=ZFS confusion in recent literature

“…The distinction between H CF (H LF ) and the conventionalH SH (H ZFS ) is clear in, e.g. the study [104] of a mononuclear high-spin Fe(II) SMM. However, the CF ¼ ZFS confusion occurs in another coauthored paper [105].…”

“…(3), however, the true ZFS parameters B m n were named as 'CF parameters'. Other pertinent comments on the papers [103][104][105] concerning usage of the Stevens operators would require separate consideration.…”