Dinuclear copper(II) complexes of two new, tetradentate (N-4), dinucleating ligands 2,5-bis((2-pyridylmethyl)thio) thiadiazole (BPMTD) and 2,5-bis(2-pyridylthio) thiadiazole (BPTD) are described. The complexes [Cut(BPMTD) (mu 2-Cl)(2)Cl-2].2CH(3)OH.0.5CH(3)CN (1), [Cu-2(BPTD)(mu(2)-X(2))(2)X(2)] (X = Cl(2), X = Br(3)), [Cu-2(BPTD)(mu(2)-OH)(NO3)(3)] (4), and [Cu(BPMTD)Cl-2](n) (5) were isolated and the X-ray crystal structures of 3 and 5 determined. 3 crystallized in the monoclinic system, space group C2/c, with a = 11.750(2) Angstrom, b = 14.235(3) Angstrom, c = 12.243(3) Angstrom, beta = 107.53(2)degrees, and Z = 4 (R = 0.031 and R(w) = 0.027). 5 crystallized in the orthorhombic system, space group Pbca, with a = 15.088(4) Angstrom, b = 13.642(8) Angstrom, c = 8.986(4) Angstrom, and Z = 4 (R = 0.058 and R(w) = 0.054). In 3 two slightly distorted square-pyramidal copper(II) centers are bridged simultaneously by the diazole (N-2) and by two halogens in an axial equatorial arrangement. A preliminary structure for 1 indicates a similarly bridged species, but Cu-Cu separations differ markedly (3.224(4) Angstrom (1); 3.556(2) Angstrom (3)), due to the different chelate ring sizes. Low room-temperature magnetic moments and variable-temperature magnetic studies (4-300 K) indicate the presence of weak antiferromagnetic exchange in 1-3 and strong antiferromagnetic exchange in 4. For the first time, the thiadiazole N-2 group is examined as a superexchange bridge between copper(II) centers and is found to propagate spin coupling less effectively than pyridazine or phthalazine (thiadiazole < phthalazine < pyridazine). Cyclic voltammograms for 1-3 in DMF are typical for one-step, two-electron, quasi-reversible redox processes associated with the Cu-2(II)/Cu-2(I) redox couple. A most unusual 1-dimensional, zigzag, polymeric dinuclear structure exists in 5, with no involvement of the thiadiazole nitrogens in bonding to copper, but with the peripheral pyridine rings linking the six-coordinate copper(II) centers, which involve an equatorial CuN2Cl2 donor set and long axial interactions with exocyclic sulfur atoms.