Four binuclear copper(II) compounds with the anionic form of the ligand 4,5-dihydro-5-oxo[1,2,4]triazolo[1,5-a]pyrimidine (5tpO−) have been isolated, their formulae being [Cu2(5tpO)4(H2O)2]·2H2O (1), [Cu2(phen)2(5tpO)2(H2O)2](NO3)2·4H2O (2), [Cu2(biim)2(5tpO)2(H2O)](ClO4)2·5.5H2O (3), and [Cu2(CH3CO2)2(5tpO)2(H2O)2] (4) (phen = 1,10-phenanthroline, biim = bisimidazole). A related mononuclear complex, [Cu(phen)2(5HtpO)2](NO3)2 (5), has also been prepared. The crystal structure of compounds 1−3 has been determined by X-ray diffraction, showing their binuclear nature with four (1) or two (2, 3) bridging 5tpO− moieties. The triazolopyrimidine ligand binds the copper atoms through N3 and N4 in compounds 1 and 3, whereas a novel binding mode through N3 and the exocyclic oxygen atom has been found in 2. The study of the solution behaviour of 2 and 5 shows the important role of the solvent in the formation of the mono or binuclear complex. The reduction of the metal centres to CuI has been observed by electrochemical methods, demonstrating that this ligand stabilizes this oxidation state. Magnetic susceptibility measurements and electronic paramagnetic resonance (EPR) spectra seem to indicate a dimeric nature also for compound 4. The antiferromagnetic interaction between the copper atoms inside the dimer is very strong for 4 (2J = −221.3 cm−1) and 1 (2J = −104.9 cm−1), fairly strong for 3 (2J = −21.6 cm−1), and very weak or non-existent for 2. The nature of the copper−copper interaction in compound 1 has been analyzed by using molecular orbital calculations.