A crystal-engineering approach to organic ferrimagnetics is reported. Coulombic energy between cationic biradical with S=1 and anionic radical with S=1/2 is a promising driving force of co-crystallizing the hetero-molecular assemblage in a controllable manner. As a cationic component of "organic salt ferrimagnetics", two kinds of nitronyl nitroxide biradicals, 2,6- and 3,5-substituted pyridine derivatives, were examined. It was predicted from semi-empirical molecular orbital calculations that both the 2,6- and 3,5-derivatives have the triplet ground states both in the neutral and cationic states. The molecular ground state of the 2,6-biradical in the neutral state was found to be triplet from magnetic susceptibility measurements, while the susceptibility and ESR measurements suggested a singlet ground state for the 3,5-biradical.