Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)(2)Cr-2(L-R(4-))](2+) ((LH4)-L-O = 1,2,4,5-tetrahydroxybenzene, (LH4)-L-OS = 1,2-dithio-4,5-dihydroxybenzene, (LH4)-L-S = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between Cr-III centers in these complexes, with exchange constants of J = -2.83(3) (L-O(4-)), -2.28(5) (L-OS(4-)), and -1.80(2) (L-S(4-)) cm(-1). Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)(2)Cr-2(L-R(3-center dot))](3+). Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) (L-O(3-center dot)), -401(8) (L-OS(3-center dot)) and -487(8) (L-O(3-center dot)) cm(-1). These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.