Spin-crossover (SCO) behaviors of Fe(II)-assembled complexes [Fe(NCX)(2)(bpa)(2)](n) (X = S, Se, and BH3; bpa: 1,2-bis(4-pyridyl)ethane) were explained by using density functional theory (DFT) calculations. The TPSSh/TZVPP method gave accurate energies to all electrons and reasonable spin-transition temperatures. Further, we revealed that the occurrence of SCO is determined by carefully controlling the local structure around the iron atom.