Herein, the synthesis and physical properties of a monovalent [Ni(dmit)(2)](-) salt (dmit(2-) = 2-thioxo-1,3-dithiole-4,5-dithiolate) containing a supramolecular cation composed of Na+ ions, dibenzo[18] crown-6 molecules, and CH3CN molecules, has been reported. The desorption/adsorption of CH3CN molecules (organic solvent molecules) from/on the [Ni(dmit)(2)](-) salt occurred while maintaining the crystalline state and could be utilized to switch the magnetic properties of the salt.