Spin crossover iron(III) complexes

Metal complexes, of which metal ions have d4–d7 electrons, may exhibit spin conversion by external stimuli, and they are called spin crossover (SCO) complexes. SCO complexes are classified into two groups depending on spin transition behavior—(i) spin transition (ST) type: the spin transition between high-spin (HS) and low-spin (LS) states occurs abruptly or discontinuously within a few Kelvin. (ii) Spin equilibrium (SE) type: the spin transition occurs gradually over a wide temperature range. Note that spin transitions in some SE type SCO complexes occur very rapidly and their transition rate is the order of 107 s−1. In this article, X-ray structural and magnetic studies on ferric SCO complexes are presented and the possible origin of the rapid spin interconversion is discussed.