Abstract The title compounds, trans(1) and cis(2) isomers, were isolated by means of QAE-Sephadex A-25 column chromatography from a reaction mixture of [Co(CO3)(en)2]Cl with l-tartaric acid. The crystal structure of 1 was determined by the X-ray diffraction study. The crystal is orthorhombic, space group C2221 with a = 12.008(1), b = 9.496(2), c = 18.524(4) Å, U = 2112.2(7) Å3, Z = 4, and final R = 0.0291. The two tartrate(2–) anions coordinate to the cobalt atom as a unidentate ligand in trans position each other. The two ethylenediamine rings are in the δδ gauche conformation. The lithium ion is tetrahedrally surrounded by two carboxylate oxygens of the tartrato(2–) ligands and two oxygens of the crystal waters. The compounds 1 and 2 were also characterized by UV-vis, NMR, and CD spectra.