Spin-crossover (SCO) complexes are the transition metal coordination compounds with the electronic configuration ranging between 3d4 and 3d7 (manganese, iron, chromium, and cobalt) in (pseudo)octahedral surroundings. The variation of some external stimuli causes the reversible phase transition from low spin (LS) diamagnetic state to high spin (HS) paramagnetic state in SCO materials. The SCO is one of the best examples of molecular electronic bistability.