Metal-centered X-ray absorption and emission spectroscopy of iron corroles: implications for ligand non-innocence

Determining the electronic structure of transition metal complexes with non-innocent ligands is challenging, both experimentally and theoretically. In this study, we investigate the electronic structure of iron corrole nitrosyl (Fe[TPC](NO), TPC = meso-triphenylcorrole) using a combination of Fe L-edge and K pre-edge X-ray absorption spectroscopy (XAS), Kβ X-ray emission spectroscopy (XES), and multiconfigurational calculations. A key debate revolves around the distribution of radical character on the ligand and the spin/oxidation state of the iron center. The experimental spectra reveal that the Fe center adopts a low-spin configuration, characterized as either Fe^II or Fe^III with strong π back-bonding and little spin polarization. Calculations identified Fe[TPC](NO) primarily as {FeNO}⁶ with a corrole^3-, where the {FeNO}⁶ unit exhibits Fe^III character. However, no localized hole was found in the iron t_2g orbital due to strong covalent mixing with NO π* orbitals. The occupation of the Fe 3d _{z ²} orbital, and thus the radical character on the corrole ligand, is highly sensitive to the axial ligand environment. This was supported by wavefunction analysis over varying Fe-NO distances and comparison with iron corrole chloride (Fe[TPC]Cl), which displayed significant radical character on the corrole ligand due to the weak axial ligand. These findings provide critical insights into ligand non-innocence and covalency in metal corrole systems, offering a foundation for understanding other highly covalent transition metal systems.