In this work, we assessed the electronic structures of two pseudotetrahedral complexes of Fe II , [Fe{(SP i Pr 2 ) 2 N} 2 ] (1) and [Fe-{(SeP i Pr 2 ) 2 N} 2 ] (2), using high-frequency and -field EPR (HFEPR) and fielddependent 57 Fe Mossbauer spectroscopies. This investigation revealed S = 2 ground states characterized by moderate, negative zero-field splitting (zfs) parameters D. The crystal-field (CF) theory analysis of the spin Hamiltonian (sH) and hyperfine structure parameters revealed that the orbital ground states of 1 and 2 have a predominant d x 2 −y 2 character, which is admixed with d z 2 (∼10%). Although replacing the S-containing ligands of 1 by their Se-containing analogues in 2 leads to a smaller |D| value, our theoretical analysis, which relied on extensive ab initio CASSCF calculations, suggests that the ligand spin−orbit coupling (SOC) plays a marginal role in determining the magnetic anisotropy of these compounds. Instead, the d x 2 −y 2 β → d xy β excitations yield a large negative contribution, which dominates the zfs of both 1 and 2, while the different energies of the d x 2 −y 2 β → d xz β transitions are the predominant factor responsible for the difference in zfs between 1 and 2. The electronic structures of these compounds are contrasted with those of other [FeS 4 ] sites, including reduced rubredoxin by considering a D 2 -type distortion of the [Fe(E−X) 4 ] cores, where E = S, Se; X = C, P. Our combined CASSCF/DFT calculations indicate that while the character of the orbital ground state and the quintet excited states' contribution to the zfs of 1 and 2 are modulated by the magnitude of the D 2 distortion, this structural change does not impact the contribution of the excited triplet states.
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