Synergistic spin transition between spin crossover and spin-Peierls-like singlet formation in the halogen-bonded molecular hybrid system: [Fe(Iqsal)2 ][Ni(dmit)2 ]⋅CH3 CN⋅H2 O.

To introduce halogen-bond interactions between a cation and an anion, a novel Fe(III) complex from iodine-substituted ligands involving a paramagnetic nickel dithiolene anion was prepared and characterized. The compound exhibited the synergy between a spin-crossover transition and a spin-Peierls-like singlet formation. The halogen-bond interactions between the iodine and the sulfur atoms stabilized the paramagnetic state of π-spins and played a crucial role in the synergistic magnetic transition between d- and π-spins. In addition, the compound showed the light-induced excited spin state trapping effect.