# Multireference Modeling Reveals the Origins of L‑Edge X‑ray Absorption Features in Photoredox-Active Nickel Complexes

## TL;DR

This paper explains the origin of a specific feature in X-ray absorption spectra of nickel catalysts using advanced computational methods.

## Contribution

The study reveals that the spectral shoulder arises from 2p to 3d transitions modulated by π* orbitals, not direct core excitations.

## Key findings

- The shoulder feature in L3-edge spectra is due to spin-free states with different spin multiplicities.
- MLCT active spaces better match experimental peak-height ratios than d–d active spaces.
- Nearby π* orbitals modulate 2p to 3d transitions rather than direct core excitations.

## Abstract

Nickel photoredox catalysts exhibit an unusual shoulder
feature
in the L3-edge X-ray absorption spectra, which has been
attributed to multiconfigurational ground-state character. We investigate
this hypothesis with multireference methods by comparing active spaces
with primarily d-orbital character (d–d) and active spaces
that replace a second shell d-orbital with a π* orbital on the
bipyridine ligand to allow for metal–ligand charge transfer
(MLCT). Through the d–d active space calculations, we find
that the splitting between the main peak and the shoulder is a result
of two spin–orbit states dominated by spin-free states with
different spin multiplicities. MLCT active spaces better capture experimental
peak-height ratios, supporting previous assignments of trends in the
shoulder intensity. Overall, our analysis suggests that the shoulder
arises from 2p to 3d transitions modulated by nearby π* orbitals
rather than direct core 2p to π* excitations. These results
demonstrate how active space composition and spin multiplicity impact
L3-edge spectral features in these multiconfigurational
transition-metal complexes.