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

High Resolution Image Download MS PowerPoint Slide Nickel photoredox catalysts exhibit an unusual shoulder feature in the L 3 -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 L 3 -edge spectral features in these multiconfigurational transition-metal complexes.