Two manganese(III) tetradentate Schiff- base dimers to which N, N'-dicyano- ,4-naphthoquinonediiminate (DCNNQI) radicals are attached have been selectively synthesized by varying the solvents used in the reactions: [Mn-2(5-MeOsaltmen) (2)(DCNNQI)(2)] center dot MeOH ( 1) and [Mn-2(5-MeOsaltmen)(2)(DCNNQI)(2)] center dot 2CH(2)Cl(2) center dot 2CH(3)CN ( 2) [5-MeOsaltmen(2-) = N, N'-(1,1,2,2-tetramethylethylene) bis(5-methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI(center dot-)) - Mn-III -(O)(2) - Mn-III -(DCNNQI(center dot-))], where -(O)(2-) is a biphenolate bridge in the out-of-plane dimerized [Mn-2(5-MeOsaltmen)(2)](2+) moiety. However, their packing arrangements are completely different. Whereas complex 1 is found to be relatively isolated, strong intermolecular dimerization of the DCNNQI moieties ( with the nearest contact being similar to 3.0 angstrom) is observed in 2, forming a one-dimensional chain of [ -Mn-III -( O)(2) - Mn-III (- DCNNQI(center dot-))(2)-](infinity). The magnetic susceptibility of 1 can be modeled with an [ S) 1/2, 2, 2, 1/2] four-spin system including strong antiferromagnetic Mn-III/ DCNNQI radical coupling (J(Mn /rad)/ k(B) = - 23 K) and ferromagnetic Mn-III/ Mn-III coupling through the biphenolate bridge ( J(Mn /Mn)/ k(B) = + 2.0 K). These interactions lead to an S-T = 3 ground state that possesses significant uniaxial anisotropy (D-S=3/ k(B) = - 2.1 K). Low-temperature ac and dc magnetic data of 1 reveal its single-molecule magnet behavior with quantum tunneling of the magnetization. By contrast, 2 possesses the diamagnetic ground state induced by dominating Mn-III - Mn-III antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or pi - pi contact along the b axis.