The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe(II)2 complexes [(Me(3)TPyA)(2)Fe-2(L-R)](n+ )(Me(3)TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: (LH2)-L-OMc = 3,6-dimethory-2,5-dihydroxo-1,4-benzoquinone, Na-2[L-NO2] = sodium 3,6-dinitro-2,5-dihydroxol,4-benzoquinone; n = 4: L-SMe2 = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electronreduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe(II )centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO2, SMe2) cm(-1). In contrast, X-ray diffraction, cyclic voltammetry, and Mossbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm(-1) for R = OMe, Cl, NO2, and SMe2, respectively. The minimal effects of substituents in the 3- and 6-positions of L-R(x-.) on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U-eff = 50 (1 ), 41 ( 1), 38(1), and 33(1) cm(-1) for R = OMe, Cl, NO2, and SMe2, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.