A one-dimensional (ID) polynuclear spin-crossover Fe-III complex, [Fe-III(acacen)(bpyp)](n)(BPh4)(n), showed a gradual and complete SC over a wide temperature range of ca. 250K between S = 5/2 and S = 1/2, where acacen = N,N'-bis(acetylacetonato)ethylenediamine, bpyp = 1,3-bis(4-pyridyl)propane, and BPh4- = tetraphenylborate. The single-crystal X-ray structures in the HS, LS, and midpoint between the HS and LS states were determined, and demonstrated that the SC may occur through the alternately arrayed HS- and LS-chains.