[Fe(hyetrz) 3 ](anion) 2 ·3H 2 O [hyetrz=4-(2′-hydroxyethyl)-1,2,4-triazole, anion=3-nitrophenylsulfonate] is a novel linear polynuclear Fe II spin-crossover compound. The low-spin to high-spin transition accompanied by a pronounced thermochromic effect occurs at 370 K in a very abrupt way. Just before this temperature, the three non-coordinated water molecules are removed. The dehydrated high-spin form remains stable down to ca. 100 K, where it transforms into a new low-spin form, implying that this material shows an apparent thermal hysteresis width of 270 K. Applications of this material are discussed.