One-dimensional (1D) coordination polymers of formula [Fe(NH 2trz)3]A·nH2O, {A = TiF62-, n = 0.5(1) and n=1 (2); A = ZrF62-, n = 0.5 (3) and n = 0 (4); A = SnF62-, n = 0.5 (5) and n = 1 (6); A = TaF72-, n = 3 (7) and n = 2.5 (8); A = GeF 62-, n = 1 (9) and n = 0.5 (10), NH2trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mössbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by 57Fe Mössbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF62- counteranion and the 1D chains, an organization which is also revealed in [Cu(NH2trz) 3]ZrF6·H2O (11). Trinuclear spin crossover compounds of formula [Fe3(NH2trz) 10(H2O)2](SbF6)6· S {S = 1.5CH3OH (12), 0.5C2H5OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T1/2 of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 ≤ V (nm3) ≤ 0.09) and for divalent anions (above V ≥ 0.11 nm3) with saturation around T1/2 = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counter-anions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived. © 2009 American Chemical Society.