Hydrazone based spin crossover complexes: Behind the extra flexibility of the hydrazone moiety to switch the spin state

Abstract Recently, ligand systems with straightforward synthesis, easily tunable and better stability are becoming an attractive research area in coordination chemistry where hydrazone based ligand systems are one of them. In spin crossover (SCO) chemistry, hydrazone based ligand systems has been grouped under the banner of imine based ligands in spite of their distinct advantages accomplished by the hydrazone moiety leading to compatibility towards multi-functional spin state switching modulated by different physico-chemical intermolecular interactions. The present focus review presents the most notable hydrazone based ligand systems having N6, N4O2, N4S2, N2O2S2 and N3O2S coordination sphere employed for constructing mono and multi nuclear Fe(II/III), Co(II) and Mn(III) complexes exhibiting an SCO event reported so far with their structural and magnetic explanations in detail. Moreover, the present review is systematically structured to provide not only to showcase the extra flexibility of hydrazone based ligand systems but also to provide a separate condensed library of these ligand systems demonstrated in the field of SCO for the first time. We hope that this focus review will lead to a divergence of these ligand systems from the general imine based ligand system counterparts which could open a new research pathway in SCO chemistry.