The electronic structure of binuclear diruthenium tetracarboxylates, in both the divalent Ru-2(II,II)(O2CR)(4) and the mixed-valent Ru-2(II,III)(O2CR)(4)X (X = anion) states is studied by means of ZINDO/S-MRCI and DFT calculations. Both methods predict a (pi*)(2)(delta*)(2) ground-state configuration for the divalent species, contrary to the (pi*)(3)(delta(*))(1) previously predicted by SCF-Xa calculations, but in agreement with magnetic measurements that show a strong Zero Field Splitting. Our ZINDO/S-MRCI calculations on compounds containing axial Ligands with different degree of pi bonding to the bimetallic center (water, chloride, carboxylate, pyridine, pyrazine), for both (II,II) and (II,III) ruthenium cores, show the important role played by the ligands. These theoretical calculations allow us to explain the differences observed in the UV/vis and resonance Raman spectra, both in solution and in the solid state, when varying the axial ligands. The ZINDO/S-MRCI calculations are also capable of solving some controversies found in the Literature, related to the assignment of the main electronic bands for both kinds of compounds. The electronic structures predicted by the DET methodology are in agreement with both the experimental evidence and the ZINDO/S calculations. Moreover, our DFT calculations provide an interpretation of the intermolecular magnetic interactions in the mixed-valent species, explaining the dependence of the antiferromagnetic coupling on the intermolecular Ru-X-Ru angle.