Mixed Metal Complexes in Solution. Thermodynamic and Spectrophotometric Study of Copper(II)-Citrate Heterobinuclear Complexes with Nickel(II), Zinc(II) or Cadmium(II) in Aqueous Solution

A thermodynamic study of CuII–MII-citrate (MII=NiII, ZnII or CdII) ternary systems has been performed by means of potentiometric measurements of hydrogen ion concentration at different temperatures (10, 25, 35 and 45°C) and at I=0.1 mol dm–3 (KNO3). The different binary and ternary systems involved have been further characterized by visible spectra and by calculating the spectra ( versus ) of all the CuII complexes. The thermodynamic data suggest strong entropic stabilization for the species under discussion. As regards the visible spectral characteristics of CuII(d-d transitions), the substitution of one CuII ion in the dimer [Cu2(cit)2H–2]4– by NiII or ZnII to form heterobinuclear [CuM(cit)2H–2]4– complexes, gives rise to a change in the visible spectrum.