AbstractComplexes with the formula [{Cu(L)}2(ox)]·2H2O (1), [Cu(HL)(ox)(H2O)] (2), [{Cu(HL)}2(ox)][Cu(ox)2]·2H2O (3) and [{Cu(HL)}2(ox)](NO3)2 (4) (HL = C7H8N4S, pyridine‐2‐carbaldehyde thiosemicarbazone) have been synthesized from Cu(L)(NO3) and oxalic acid while modulating the stoichiometries and the pH of the media. Single crystal X‐ray diffraction studies show that the structures of 1 and 4 contain dinuclear entities with distorted square pyramidal copper(II) ions. The dinuclear complexes are centrosymmetric in 1 and non‐centrosymmetric in 4. The metal centers are coordinated to the NNS atoms from the tridentate thiosemicarbazone ligand and to two oxygen atoms from the oxalato coligand. The latter ligand exhibits tetradentate behavior and acts as a bridge between the copper(II) ions. The thiosemicarbazone ligand is either present in the anionic (1) or the neutral (4) form. Compound 2 is composed of monomeric species containing square pyramidal copper(II) ions that are coordinated to the thiosemicarbazone ligand, one water molecule and one monodentate oxalato coligand. It is important to note that the oxalato is highly twisted in this complex, the O(1)−C(8)−C(9)−O(3) dihedral angle between the carboxylate groups is 79.9(4)°. Compound 3 contains both the dinuclear cationic species, analogous to those described for compound 4, and square planar (bisoxalato)copper(II) anions. Strong hydrogen bonds and π‐π stacking are present in these complexes. Structural and spectroscopic results suggest the presence of relevant ligand‐to‐metal charge transfers in these compounds. The EPR spectra of 1, 2 and 4 exhibit rhombic symmetry. Magnetic measurements on 1 and 4 show antiferromagnetic and ferromagnetic coupling, respectively. The susceptibility data were fitted to the Bleaney−Bowers equation for copper(II) dinuclear complexes. The J/k values obtained are −6.20 and +7.73 K for compounds 1 and 4, respectively. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)