A mononuclear FeII complex that shows a high‐spin (S=2) paramagnetic behavior at all temperatures (with standard temperature‐scan rates, ≈1 K min−1) has, in fact, a low‐spin (S=0) ground state below 100 K. This low‐spin state is not easily accessible due to the extremely slow dynamics of the spin‐crossover process—a full relaxation from the metastable high‐spin state to the low‐spin ground state takes more than 5 h below 80 K. Bidirectional photo‐switching of the FeII state is achieved reproducibly by two selective irradiations (at 530–590 and 830–850 nm). The slow dynamics of the spin‐crossover and the strong structural cooperativity result in a remarkably wide 95‐K hysteresis loop induced by both temperature and selected light stimuli.

Three isostructural complexes with the formula [Fe(L 5Me )(NCE) 2 ]: L 5Me = N,N′-bis(5-methyl-2-pyridylmethyl)ethane-1,2-diamine and E = S (1-S), E = Se (1-Se), E = BH 3 (1-BH 3 ) have been synthesized and characterized by single-crystal x-ray diffraction, magnetic susceptibility and DSC studies. All the three derivatives are spin crossover (SCO) active, showing complete one-step spin conversion. The SCO midpoint temperatures (T 1/2 ) are 193 K for 1-S, 226 K for 1-Se, and 330 K for 1-BH 3 , which are among the highest values for the homologous Fe(II)-NCE complexes with comparable tetradentate ligands. The almost linear Fe-N ≡ C(E) angles are consistent with the strong ligand field (LF) strength imposed by these NCE − co-ligands. Strong hydrogen-like bonding N-H…E was observed to connect the molecules into 2D supramolecular sheets parallel to the bc plane. However, such supramolecular interaction is not sufficient enough to transmit strong cooperativity. A discussion on the factors governing the LF strength and the cooperativity has been made, based on the comparison of analogous complexes and also based on UV-vis spectroscopy studies of the Ni(II) complexes.

A mononuclear FeII complex that shows a high‐spin (S=2) paramagnetic behavior at all temperatures (with standard temperature‐scan rates, ≈1 K min−1) has, in fact, a low‐spin (S=0) ground state below 100 K. This low‐spin state is not easily accessible due to the extremely slow dynamics of the spin‐crossover process—a full relaxation from the metastable high‐spin state to the low‐spin ground state takes more than 5 h below 80 K. Bidirectional photo‐switching of the FeII state is achieved reproducibly by two selective irradiations (at 530–590 and 830–850 nm). The slow dynamics of the spin‐crossover and the strong structural cooperativity result in a remarkably wide 95‐K hysteresis loop induced by both temperature and selected light stimuli.