Electronic Structure of Tetrahedral, = 2, [Fe{(EPPr)N}], E = S, Se, Complexes: Investigation by High-Frequency and -Field Electron Paramagnetic Resonance, Fe Mössbauer Spectroscopy, and Quantum Chemical Studies.

In this work, we assessed the electronic structures of two pseudotetrahedral complexes of Fe, [Fe{(SPPr)N}] ) and [Fe{(SePPr)N}] (), using high-frequency and -field EPR (HFEPR) and field-dependent Fe Mössbauer spectroscopies. This investigation revealed = 2 ground states characterized by moderate, negative zero-field splitting (zfs) parameters . The crystal-field (CF) theory analysis of the spin Hamiltonian (sH) and hyperfine structure parameters revealed that the orbital ground states of and have a predominant d character, which is admixed with d (∼10%). Although replacing the S-containing ligands of by their Se-containing analogues in leads to a smaller || value, our theoretical analysis, which relied on extensive 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 → d excitations yield a large negative contribution, which dominates the zfs of both and , while the different energies of the d → d transitions are the predominant factor responsible for the difference in zfs between and . The electronic structures of these compounds are contrasted with those of other [FeS] sites, including reduced rubredoxin by considering a -type distortion of the [Fe(E-X)] 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 and are modulated by the magnitude of the distortion, this structural change does not impact the contribution of the excited triplet states.