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

## TL;DR

This study uses X-ray spectroscopy and calculations to understand how iron and ligands share electrons in iron corrole complexes, revealing how their interactions affect the electronic structure.

## Contribution

The work provides new insights into ligand non-innocence and covalency in iron corrole systems using advanced X-ray and computational methods.

## Key findings

- The iron center in Fe[TPC](NO) is low-spin with strong π back-bonding and minimal spin polarization.
- Radical character on the corrole ligand is sensitive to the axial ligand environment.
- Bidirectional charge transfer occurs between the metal and ligand, influencing covalency and 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 FeII or FeIII with strong π back-bonding and little spin polarization. Calculations identified Fe[TPC](NO) primarily as {FeNO}6 with a corrole3−, where the {FeNO}6 unit exhibits FeIII character. However, no localized hole was found in the iron t2g orbital due to strong covalent mixing with NO π* orbitals. The occupation of the Fe 3dz2 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.

Metal-centered X-ray spectroscopy and multiconfigurational theory reveal bidirectional charge transfer in iron corroles, with ligand-to-metal and metal-to-ligand interactions governing covalency and ligand non-innocence.