The ion pair charge-transfer (IPCT) complexes (C14H12N2)[Cu(opba)]. 3H(2)O (1) and Na-2(C12H12N2)[Cu(opba)](2). 4H(2)O (2), opba o-phenylenebis (oxamate), have been is elated. Both compounds crystallize in the triclinic space group P (1) over bar. Crystal data for the compounds 1, a = 7.3216(13) Angstrom, b = 10.1756(9) Angstrom, c 16.4890(16) Angstrom, alpha 107.786(8)degrees, beta = 94.79(1)degrees, gamma = 104.16(1)degrees, V = 1117.4(3) Angstrom(3), Z = 2, R = 0.033 for 5332 observed reflections with I > 3 sigma(I); 2, a = 6.732(3) Angstrom, b = 11.169(3) Angstrom, c = 12.126(2) Angstrom, alpha = 111.01(2)degrees, beta = 102.71(3)degrees, gamma = 93.57(3)degrees, V = 820.1(5) Angstrom(3), Z = 2, R = 0.065 for 3354 observed reflections with I > 3 sigma(I). The crystal structures consist of alternated anion complex and diquaternary cation layers pi-pi interacting through the corresponding aromatic rings. The metallic layers contain the [Cu(opba)](2)(4-) dimeric group where the copper(II) ions are in square-pyramidal environment. For both compounds an ion-pair charge-transfer (IPCT) band is observed at 517 nm. Their thermal behavior has also been explained on the basis of the different crystal arrangements. The dimeric nature of compound 1 is clearly confirmed by the observation of temperature-dependent ESR transitions between its singlet and triplet states. The \J\ value, determined from the positions of these transitions, is 0.085 cm(-1) at room temperature and increases notably with decreasing temperature being 0.14 cm(-1) at 4.2 K. Comparison of these exchange constants with those observed for other carboxylate-bridged copper compounds allow us to deduce that the temperature dependence of J is due to lattice shrinkage leading to appreciable change of the Cu-O-ap distance. The ESR spectrum of compound 2 is characteristic of an axial g-tensor with g(parallel to) = 2 178 and g(perpendicular to) = 2.049, showing a weak half-field Delta M-s = 2 transition in agreement with the dimeric nature of the complex. The thermal evolution of the magnetic susceptibility corresponds to a compound with weak antiferromagnetic interactions. The best fit of the magnetic data to a dimer S = 1/2 model gives J = -0.8 cm(-1), in good agreement with the topology of the bridging unit with the magnetic orbitals in parallel planes and a Cu-O-Cu angle of 95.4 degrees.