A Spectroscopic Criterion for Identifying the Degree of Ground-Level Near-Degeneracy Derived from Effective Hamiltonian Analyses of Three-Coordinate Iron Complexes

The fascinating magnetic and catalytic properties of coordinatively unsaturated 3d metal complexes are a manifestation of their electronic structures, in particular their nearly doubly or triply degenerate orbital ground levels. Here, we propose a criterion to determine the degree of degeneracy of this class of complexes based on their experimentally accessible magnetic anisotropy (parametrized by the electron spin g- and zero-field splitting (ZFS)-tensors). The criterion is derived from a comprehensive spectroscopic and theoretical study in the trigonal planar iron(0) complex, [(IMes)Fe(dvtms)] (IMes = 1,3-di(2′,4′,6′-trimethylphenyl)imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane, 1). Accurate ZFS-values (D = +33.54 cm^–1, E/D = 0.09) and g-values (g_∥ = 1.96, g_⊥ = 2.45) of the triplet (S = 1) ground level of complex 1 were determined by complementary THz-EPR spectroscopy and SQUID magnetometry. In-depth effective Hamiltonian (EH) analyses coupled to wave-function-based ab initio calculations show that 1 features a ground level with three energetically close-lying orbital states with a “two-above-one” energy pattern. The observed magnetic anisotropy results from mixing of the two excited electronic states with the ground state by spin–orbit coupling (SOC). EH investigations on 1 and related complexes allowed us to generalize this finding and establish the anisotropy of the g- and ZFS-tensors as spectroscopic markers for assigning two- or three-fold orbital near-degeneracy.