Optical, Magnetic and Structural Properties of the Spin‐Crossover Complex [Fe(btr)₂(NCS)₂]·H₂O in the Light‐Induced and Thermally Quenched Metastable States

[Fe(btr)2(NCS)2]·H2O [btr=4,4′‐bis(1,2,4‐triazole)] is thearchetype of highly cooperative and low‐dimensional spin‐crossover complexes, which exhibit low‐spin (LS) to high‐spin (HS) light‐induced conversion at very low temperature. The structural reorganizations related to the light‐induced and thermally induced LS–HS transitions were characterized by single‐crystal X‐ray diffraction below the relaxation temperature (T = 15K LIESST) and at 130K within the thermal hysteresis loop. We show that the LIESST and thermal spin transitions lead to the same structural variations, namely an elongation of the Fe–N bonds by 0.18Å (Fe–NNCS) and 0.20 Å (Fe–Nbtr), on going from LS to HS, together with a reorientation of the NCS group by nearly 13°. The atomic displacement amplitudes, derived from the crystal structures, indicate lattice vibration modes of larger amplitudes and correlatively lower vibration frequencies in the HS state. The deformation of the crystal lattice as a function of temperature and laser excitation was quantitatively analyzed in terms of the HS and LS thermal‐expansion (αHS and αLS) and spin‐transition spontaneous‐strain (ϵ) tensors. The eigendirections and eigenvalues of the α and ϵ tensors correlate well with the weak and strong interactions in the solid and are responsible for the high cooperativity and low‐dimensional behaviour. Magnetic and spectroscopic measurements were performed in all the different spin states and related to the structural findings