Complexes of ethylenediamine‘,N-tetraacetanilide (edtan) with cobalt (II), nickel (II) and copper (II) in the solid state and in solution are reported for the first time. Thermodynamic data (stability constant, and derived Gibbs energy, enthalpy and entropy) for the 1:1 complexation of edtan with the metal ions at 298.15 K in water-saturated butan-l-ol gave the selectivity sequence log10Ks: Ni2+, 4.65 ±0.02; Cu2+, 4.41 ±0.01; Co2+, 4.18 ±0.04 as found from microcalorimetric titration studies. The entropies suggested that the structure of the 1:1 complex with copper (II) contains fewer chelate rings than those for nickel (II) and cobalt (II) (ΔCS°: Cu, -19.3; Co, 5.7; Ni, 3.9 J mol−1 K−1). Solid complexes of the metal ions with edtan and perchlorate as the counter anion were prepared. For each a complex with a 1:1 metal : Edtan stoichiometry with non-coordinated perchlorate (ir) was isolated. Subsequent X-ray crystallographic research has shown that [Cu(edtan)(H2O)][C1O4]2.1.5H2O has a six-coordinate Cu center with edtan acting as a pentadentate ligand (2N, 30) with the coordination sphere completed by an oxygen atom from water. Conversely and in striking contrast to the Cu complex, in [Co(edtan)(H2O)][C1O4]2.H2O.1/2C2H5OH the Co center is seven-coordinate with hexadentate edtan (2N, 4O) and one coordinated water molecule. There is thus an excellent confirmation of the results obtained from the microcalorimetric study in that edtan forms four chelate rings to Cu but five to Co in the solid state. The ability of the ligand to extract metal ions from water to the water-saturated butan-l-ol phase was assessed from distribution data as a function of the aqueous phase hydrogen ion concentration and of the ligand concentration in the organic phase. The data showed that Cu2+ is selectively extracted over a wide range of aqueous phase hydrogen ion concentrations. © 1995 IUPAC