A potential bridging triazole-based ligand, atrz (trans-4,4'-azo-1,2,4-triazole), is chosen to serve as building sticks and incorporated with a spin crossover metal center to form a metal organic framework. Coordination polymers of iron(II) with the formula [Fe(mu-atrz)(3)]X-2 center dot 2H(2)O (where X = ClO4- (1 center dot 2H(2)O) and BF4- (2 center dot 2H(2)O)) in a 3D framework and [Fe(mu-atrz)(mu-pyz)(NCS)(2)]center dot 4H(2)O (3.4H(2)O) in a 2D layer structure were synthesized and structurally characterized. The magnetic measurements of 1 center dot 2H(2)O and 2 center dot 2H(2)O reveal spin transitions near room temperature; that of 3 exhibits an abrupt spin transition at similar to 200 K with a wide thermal hysteresis, and the spin transition behavior of these polymers are apparently correlated with the water content of the sample. Crystal structures have been determined both at high spin and at low spin states for 1 center dot 2H(2)O, 2 center dot 2H(2)O, and 3 center dot 4H(2)O. Each iron(II) center in 1 center dot 2H(2)O and 2 center dot 2H(2)O is octahedrally coordinated with six mu-atrz ligands, which in turn links the other Fe center forming a strong three-dimensional (3D) network; counteranion and water molecules are located in the voids of the lattice. The FeN6 octahedron of 3 center dot 4H(2)O is formed with two atrz, two pyrazine (pyz) ligands, and two NCS- ligands, where the ligands atrz and pyz are bridged between iron centers forming a 2D layer polymer. A zigzag chain of water molecules is found between the layers, and there is a distinct correlation between the thermal hysteresis with the amount of water molecules the exist in the crystal.