Solvent-free spin crossover Fe-II complex fac-[Fe-II(HLn-Pr)(3)]Cl center dot PF6 was prepared, where HLn-Pr denotes 2-methylimidazol-4-yl-methylideneamino-n-propyl. The magnetic susceptibility measurements at scan rate of 0.5 K min(-1) showed two successive spin transition processes consisting of the first spin transition T-1 centered at 122 K (T-1 up arrow = 127.1 K, T-1 down arrow = 115.8 K) and the second spin transition T-2 centered at ca. 105 K (T-2 up arrow = 115.8 K, T-2 down arrow = 97.2 K). The magnetic susceptibility measurements at the scan rate of 2.0, 1.0, 0.5, 0.25, and 0.1 K min(-1) showed two scan speed dependent spin transitions, while the Mossbauer spectra detected only the first spin transition T-1. The crystal structures were determined at 160, 143, 120, 110, 95 K in the cooling mode, and 110, 120, and 130 K in the warming mode so as to follow the spin transition process of high-spin HS -> HS(T-1) -> HS(T-2) -> low-spin LS -> LS(T-2) -> LS(T-1) -> HS. The crystal structures at all temperatures have a triclinic space group P (1) over bar with Z = 2. The complex-cation has an octahedral N-6 coordination geometry with three bidentate ligands and assume a facial-isomer with Delta- and Lambda-enantimorphs. Three imidazole groups of fac-[Fe-II(HLn-Pr)(3)](2+) are hydrogen-bonded to three Cl- ions. The 3:3 NH(imidazole)center dot center dot center dot Cl- hydrogen-bonds form a stepwise ladder assembly structure, which is maintained during the spin transition process. The spin transition process is related to the structural changes of the FeN6 coordination environment, the order-disorder of PF6- anion, and the conformation change of n-propyl groups. The Fe-N bond distance in the HS state is longer by 0.2 angstrom than that in the LS state. Disorder of PF6- anion is not observed in the LS state but in the HS state. The conformational changes of n-propyl groups are found in the spin transition processes except for HS --> HS(T-1) -> HS(T-2).