he photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2 (1), [Fe(L2)2][BF4]2 (2), [Fe(L2)2][ClO4]2 (3), [Fe(L3)2][BF4]2 (4), [Fe(L3)2][ClO4]2 (5) and [Fe(L4)2][ClO4]2 (6) (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds 1–6 display a complete thermal spin transition centred between 200–300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS - > LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T1/2: T(LIESST) = T0– 0.3T1/2. T0 for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms of 5 and 6, the light-induced high-spin state of 5, and the low-spin complex [Fe(L4)2][BF4]2 (7), are described.< Leer menos