Crystal structures and magnetic properties were determined for two novel polymorphs of the complex [H(2)DABCO]-[Ni(mnt)(2)] [(H(2)DABCO)(2+) = diprotonated 1,4-diazabicyclo[2.2.2]octane; mnt(2-) = maleonitriledithiolate]. For each polymorph, anions form a layered structure in which two kinds of dimers were observed. The adjacent anionic sheets are held together by cations via H-bonding interactions between protons of cations and CN groups of anions. Two polymorphs possess spin bistability; namely, upon cooling, a magnetic transition happens at around 120 K with about 1 K hysteresis on heating for the alpha phase and at 112 K with about 10 K hysteresis for the beta phase. Above the transition, the magnetic behaviors of two polymorphs can be approximately interpreted by a singlet-triplet model of an antiferromagnetically coupled S = (1)/(2) dimer, which is supported by the crystal structures and spin dimer analyses based on extended Huckel molecular orbital calculations.