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

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.