t-{Fe(abpt)(2)[N(CN)(2)](2)} [abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] is an intriguing spin-crossover system that crystallizes in two polymorphs. Polymorph A is paramagnetic; its crystal structure consists of a single molecule located at the center of inversion symmetry. Polymorph B, on the other hand, exhibits a rather complicated two-step-like spin transition; its crystal structure consists of two symmetry-independent molecules. The crystal structure of polymorph B has been derived in the different spin states: above the high-temperature step (300 K), between the two steps (90 K), below the incomplete low-temperature step (50 K), in the light-induced metastable state (15 K), in the thermally quenched metastable state (15 K), and after relaxation from the quenched state (15 K). The correlation between the structure and magnetic properties is precisely established, allowing the complicated magnetic behavior of polymorph B to be well understood. A unique order-disorder phase transition, resulting in a modulation of the metastable state structures, is detected for the first time on such spin-transition compounds. The modulation of the structure originates from a particular ordering of the dicyanamide ligand at one of the two Fe sites.

Two new polymorphs (C and D) of [t-Fe(abpt)(2)(NCS)(2)] are characterized, a commensurate modulated structure with the c axis tripled in the thermally quenched metastable high spin state at 25 K is found in polymorph C.

Two polymorphs of mononuclear six-coordinate iron(II) spin-crossover complex trans-[Fe(tzpy)(2)(NCS)(2)] (tzpy = 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) (1) were isolated and structurally characterized. According to the thermally dependent magnetic measurements, polymorph A undergoes a gradual spin transition from a paramagnetic high-spin state ((5)T(2), S = 2, HS-1) above 200 K to a diamagnetic low-spin state ((1)A(1), S = 0, LS-1) below 120 K, whereas polymorph B shows an abrupt spin transition with T(1/2) at 102 K. Molecular and crystal structures of polymorph A in the HS-1 and LS-1 states were studied at 300 and 40 K, respectively. Significant differences in Fe-N distances and coordination geometries of Fe were found between the two spin states, as expected. Light-induced excited spin state trapping (LIESST) was observed upon irradiating the crystal with 532 nm laser light at 40 K, whereupon a metastable high-spin state (HS-2) was formed; the molecular and crystal structure of this metastable state were investigated by a pump and probe method because of its relatively fast relaxation. The electronic configuration of the Fe center in the HS-1, LS-1, and LIESST (HS-2) states were further confirmed by Fe K- and L-edge absorption spectroscopy. In addition, the C[triple bond]N stretching frequency on the ligand can also be followed through the spin transition. The excitation and relaxation process concerning such metastable state were followed by the C[triple bond]N stretching frequency and magnetic susceptibility measurements in the temperature ranges 15-55 K and 5-80 K, respectively. The structure and electronic configuration of the LIESST state of polymorph A were firmly established by X-ray diffraction, X-ray absorption, infrared absorption, and magnetic measurements. A single-crystal-to-single-crystal transition through irradiation was demonstrated. The changes in structure and electronic configuration as a result of the spin transition are believed to occur concurrently.

A water‐containing iron(III) complex, [FeIII(SalEen)2]ClO4·0.5H2O [1, SalEen = N‐ethyl‐N‐(2‐aminoethyl)salicylaldiminate], and an unsolvated complex, [FeIII(SalEen)2]ClO4 (2), are synthesized and structurally characterized. Complex 1 crystallizes in a polar orthorhombic space group Fdd2, whereas 2 crystallizes in a monoclinic space group P21/c. The presence or absence of water molecules results in different molecular packings, which give rise to different magnetic properties. The temperature dependent magnetic measurements show very gradual spin transitions for both 1 and 2; the iron(III) center is in the high‐spin (HS) state with S = 5/2 at 350 K and is in the low‐spin (LS) state with S = 1/2 below 30 K for 1 (T1/2 = 155 K) and at 120 K for 2 (T1/2 = 230 K). The crystal structures of the HS and LS states for both 1 and 2 are characterized. In addition, the structure of 1 at 150 K is also investigated. The Fe–N and Fe–O bond lengths are 0.14 and 0.03 Å shorter in the LS state than those in the HS state. The octahedral distortion in the iron(III) coordination geometry, expressed as Σ and Θ parameters, is slightly larger in the HS than in the LS state.