Tracing Structural Distortions and Orbital Population During Spin‐Crossover in Transition Metal Complexes via Ligand K‐edge Spectroscopy

The degree of metal–ligand covalency in transition metal complexes directly impacts the energetic order of their excited states. Assigning their role in ultrafast spin‐crossover dynamics demands their unambiguous characterization. A prominent example for a third row spin‐crossover complex is iron‐(II)‐tris(bipyridine), which efficiently relaxes into its lowest quintet state on sub‐picosecond time‐scales subsequent to photo‐excitation. It has been established that triplet states are involved as intermediates in the decay cascade. We investigate these dynamics by combining femtosecond K‐edge soft x‐ray absorption and resonant inelastic x‐ray scattering to track electronic and structural changes utilizing the nitrogen transition. We unambiguously identify a triplet state population on a 400 fs time‐scale through occupation redistribution within the subset of Fe‐d orbitals, accompanied by signatures of Fe─N bond elongation.