Metal-Centered X-ray Absorption and Emission Spectroscopy of Iron Corroles: Implications for Ligand Non-Innocence †

Abstract

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 backbonding and little spin polarization. Calculations identified Fe[TPC](NO) primarily as {FeNO} 6 with a corrole 3 -, where {FeNO} 6 unit exhibits ferric 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 3dz 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 innocence and covalency in metal corrole systems, offering a foundation for understanding other highly covalent transition metal systems.