EPR spectra and structures of spin-variable iron(III) complexes

The structures of the spin-variable iron(III) complexes Fe(4-OCH₃-SalEen)₂X and FeL′₂X (L′ is oxysalicylidene-N′-ethyl-N-ethylenediamine; X = PF₆, NO₃, and SCN) were examined from their EPR spectra. The coordination units (CUs) of these complexes are a high-symmetry octahedron with a slight tetrahedral distortion or a low-symmetry pseudooctahedron with considerable tetragonal and orthorhombic distortions. When the complex passes from the high-spin to low-spin state, its CU changes. The energy level splittings caused by various types of distortions were estimated from the EPR data. The distortion of the CU depends on the outer-sphere anion. The magnetic resonance parameters of the complexes were analyzed. The fine structure parameters of the EPR spectra of high-spin complexes (the ground-state term is $^6 A_{1_g }$ ) depend on the CU distortion and the covalent bonding. The spin-orbital coupling makes an appreciable second-order contribution to the expressions for the Zeeman coupling parameters of low-spin complexes (the ground-state term is $^2 T_{2_g }$ ).