An ion-pair compound 1,6-bis(1-methylimidazolium)hexane bis(maleonitrio-dithiolato)nickelate was synthesized and structurally characterized. This compound crystallizes in the triclinic space group P-1, and the anions form the alternating stacks in the crystal. Reversible three-step solid-to-solid phase transitions occur at ~210, ~320 and ~432K upon cooling. The entropy change of phase transition was estimated to be 25.4J⋅K −1 ⋅mol −1 and 9.97J⋅K −1 ⋅mol −1 for the transitions at ~320 and ~432K, respectively. The hysteresis effect was observed in the χ m –T plot for the transitions at ~210 and ~320K. The mechanisms of transitions are discussed based on the crystal structure and thermal analyses.