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 <i>g</i>- 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, <b>1</b>). Accurate ZFS-values (<i>D</i> = +33.54 cm^-1, <i>E</i>/<i>D</i> = 0.09) and <i>g</i>-values (<i>g</i> _∥ = 1.96, <i>g</i> _⊥ = 2.45) of the triplet (<i>S</i> = 1) ground level of complex <b>1</b> were determined by complementary THz-EPR spectroscopy and SQUID magnetometry. In-depth effective Hamiltonian (EH) analyses coupled to wave-function-based <i>ab initio</i> calculations show that <b>1</b> 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 <b>1</b> and related complexes allowed us to generalize this finding and establish the anisotropy of the <i><b>g</b></i> - and ZFS-tensors as spectroscopic markers for assigning two- or three-fold orbital near-degeneracy.