text: SCO was first observed in 1931 by Cambi et al. who discovered anomalous magnetic behavior for the tris(N,N-dialkyldithiocarbamatoiron(III) complexes. The spin states of these complexes were sensitive to the nature of the amine substituents. In the 1960s, the first Co SCO complex was reported. Magnetic measurements and Mössbauer spectroscopic studies established the nature of the spin transition in iron(II) SCO complexes. Building on those early studies, there is now interest in applications of SCO in electronic and optical displays.