Monovalent cations (M+ = Na+, K+, Rb+, and NH4+) and 12-crown-4 were assembled to new supramolecular cation (SC+) structures of the M+(12-crown-3)(n) (n = 1 and 2), which were incorporated into the electrically conducting Ni(dmit)(2) salts (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate), The Na+, K+, and Rb+ salts are isostructural with a stoichiometry of the M+(12-crown-3)(2)[Ni(dmit)(2)](4), while the NH4+ salt has a stoichiometry of NH4+(12-crown-4) [Ni(dmit)(2)](3)(CH3CN)(2). The electrical conductivities of the Na+, K+, Rb+, and NH4+ salts at room temperature are 7.87, 4.46, 0.78, and 0.14 S cm(-1), respectively, with a semiconducting temperature dependence. The SC+ structures of the Na+, K+, and Rb+ salts have an ion-capturing sandwich-type cavity of M+(12-crown-4)(2), in which the M+ ion is coordinated by eight oxygen atoms of the two 12-crown-3 molecules. On the other hand, the NH4+ ion is coordinated by four oxygen atoms of the 12-crown-4 molecule. Judging from the M+-O distances, thermal parameters of oxygen atoms, and vibration spectra, the thermal fluctuation of the Na+(12-crown-4)(2) structure is larger than those of K+(12-crown-4)2 and Rb+(12-crown-4)2. The SC+ unit with the larger alkali metal cation gave a stress to the Ni(dmit)(2) column, and the SC+ structure changed the pi-pi overlap mode and electrically conducting behavior.