Spin crossover in iron(II) tris(diimine) and bis(terimine) systems

Tris(diimine) and bis(terimine) iron(II) complex salts constitute one of the major classes of spin crossover systems. Both electronic and structural modifications can be made so as to bring the ligand field of the parent imines, 2,2'-bipyridine, 1,10-phenanthroline and 2,2':6',2"-terpyridine into the crossover range with some degree of confidence. The resulting imine systems are considered and classified according to their structural types. Among the many crossover systems in this class are several which display a high degree of cooperativity in the solid state, and the incorporation of hydrogen-bonding sites into the ligand structure also strongly influences many solid-state properties.