Investigating the photoinduced electronic and structural response of bistable molecular building blocks incorporating transition metals in solution phase constitutes a necessary stepping stone for steering their properties toward applications and performance optimizations. This work presents a detailed X-ray transient absorption (XTA) spectroscopy study of a prototypical spin crossover (SCO) complex [Fe^II(mbpy)₃]²⁺ (where mbpy = 4,4′-dimethyl-2,2′-bipyridine) with an [Fe^IIN₆] first coordination shell in water (H₂O) and acetonitrile (CH₃CN). The unprecedented data quality of the XTA spectra together with the direct fitting of the difference spectra in k space using a large number of scattering paths enables resolving the subtle difference in the photoexcited structures of an Fe^II complex in two solvents for the first time. Compared to the low spin (LS) ¹A₁ state, the average Fe–N bond elongations for the photoinduced high spin (HS) ⁵T₂ state are found to be 0.181 ± 0.003 Ã… in H₂O and 0.199 ± 0.003 Ã… in CH₃CN. This difference in structural response is attributed to ligand–solvent interactions that are stronger in H₂O than in CH₃CN for the HS excited state. Our studies demonstrate that, although the metal center of [Fe^II(mbpy)₃]²⁺ could have been expected to be rather shielded by the three bidentate ligands with quasi-octahedral coordination, the ligand field strength in the HS excited state is nevertheless indirectly affected by solvation effects that modifies the charge distribution within the Fe–N covalent bonds. More generally, this work highlights the importance of including solvation dynamics in order to develop a generalized understanding of the spin-state switching at the atomic level.

For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/lowspin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specifi c deformation in the photoinduced high-spin state of solvated [Fe(terpy)₂ ]²⁺ , a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal− ligand bonds. The correlated changes in Fe−N_Axial , Fe− N_Distal , and bite angle N_Distal− Fe− N_Axial  extracted from the measurements are in very good agreement with those predicted by DFT calculations in D_2d  symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order.