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We report three self‐assembled iron complexes that comprised an anti‐parallel open form (o‐Lanti), a parallel open form (o‐Lsyn), and a closed form (c‐L) of diarylethene conformers. Under kinetic control, FeII2(o‐Lanti)3 was isolated, which exhibited a dinuclear structure with diamagnetic properties. Under light‐irradiation control, FeII2(c‐L)3 was prepared and exhibited paramagnetism and spin‐crossover behaviour. Under thermodynamic control and in the presence of indispensable [FeIII(Tp*)(CN)3]−, FeII2(o‐Lanti)3 and FeII2(c‐L)3 transformed into tetranuclear FeIII2FeII2(o‐Lsyn)2, which exhibited complete spin‐crossover behaviour at T1/2=353 K. [ABSTRACT FROM AUTHOR]

A neutral mononuclear FeIII complex [FeIII(H-5-Br-thsa-Me)(5-Br-thsa-Me)]⋅H2O ( 1; H2-5-Br-thsa-Me=5-bromosalicylaldehyde methylthiosemicarbazone) was prepared that exhibited a three-step spin-crossover (SCO) with symmetry breaking and a 14 K hysteresis loop owing to strong cooperativity. Two ordered intermediate states of 1 were observed, 4HS-2LS and 2HS-4LS, which exhibited reentrant phase-transition behavior. This study provides a new platform for examining multistability in SCO complexes. [ABSTRACT FROM AUTHOR]