2-Methylimidazol-4-yl-methylideneamino-2-ethylpyddine (abbreviated as HLMe) is the 1:1 condensation product of 2-methyl-4-formylimidazole and 2-aminoethylpyridine and functions as a bidentate ligand to the iron(II) ion to produce the 3:1 complexes together with anions, [Fe(HLMe)(3)]X-2 (X = PF6 (1), ClO4 (2), BF4 (3)). The magnetic susceptibilities, differential scanning calorimetric measurements, and Mossbauer spectral measurements demonstrated that complexes 1, 2, and 3 showed a steep one-step spin crossover (SCO) between the high-spin (HS, S=2) and low-spin (LS, S = 0) states with small thermal hysteresis. Three complexes have an isomorphous structure and are crystallized in the same monoclinic space group, C2/c, both in the HS and LS states. The iron(II) ion has the octahedral coordination geometry of a facial isomer with N-6 donor atoms of three bidentate ligands, in which an imidazole and an imine nitrogen atom per ligand participate in the formation of the coordination bond, but the pyridine nitrogen is free from coordination. The complex cation fac-[Fe(HLMe)(3)](2+) is a chiral species with a Delta or Lambda isomer, and the adjacent Delta and Lambda isomers are linked alternately by an intermolecular imidazole-pyridine NH center dot center dot center dot N hydrogen bond to produce an achiral 1D chain. The two remaining imidazole moieties per complex are hydrogen-bonded to the anions that occupy the space among the chains. The SCO profile becomes steeper with the decrease of the anion size (73.0 angstrom(3) for PF6-, 54.4 angstrom(3) for ClO4-, and 53.4 angstrom(3) for BF4-). The SCO transition temperature T-1/2 of the PF6 (1), ClO4 (2), and BF4 (3) salts estimated from the magnetic susceptibility measurements are (T-down arrow = 151.8 K, T-up arrow = 155.3 K), (T-down arrow = 184.5 K, T-up arrow = 186.0 K), and (T-down arrow = 146.4 K, T-up arrow = 148.2 K), respectively, indicating that the T-1/2 value is not in accord with the anion size.