mu-Chloro-mu-[2,5-bis(2-pyridyl)-1,3,4-thiadiazole] aqua chlorocopper(II) dichlorocopper(II) is the first characterized dimeric complex of a transition metal and this hetero ligand [C12H10Cl4Cu2N4OS; triclinic; space group P (1) over bar; a = 9.296(3) Angstrom, b = 9.933(3) Angstrom, c = 10.412(3) Angstrom; alpha = 79.054(5)degrees, beta = 82.478(6)degrees, gamma = 67.099(5)degrees; Z = 2 at room temperature]. The Cu(II) ions are bridged by the N-N thiadiazole bond and a chloride ion [Cu1-Cu2 = 3.7800(8) Angstrom]. Thermogravimetric analysis shows this structure to be stable at temperatures up to 348 K. At higher temperatures, the successive loss of a water molecule and one chloride of the dimeric unit is identified. From room temperature to 125 K, half of the Cu2+ ions are progressively engaged in intermolecular dinuclear antiferromagnetic exchanges, while the other half remain paramagnetic. At lower temperatures, both susceptibility and electron paramagnetic resonance measurements show the paramagnetic-only couplings of this half of the Cu2+ ions, involving a singlet ground state for interacting Cu2+. This unusual behavior has been satisfactorily explained on the basis of intermolecular Cu-Cu interactions (J = -180 cm(-1)), involving the magnetic d(z)(2) orbital perpendicular to the molecular plane, on which are seen the conjugate effects of the bridging chloride and the planar thiadiazole. It is noteworthy that the behavior of the title compound is original, compared to the systematic in-plane intramolecular antiferromagnetic coupling of other thiadiazole-containing binuclear complexes.