Calorimetric and Spectrophotometric Studies of Chloro Complexes of Nickel(II) and Zinc(II) Ions in N,N-Dimethylformamide

Abstract

Abstract Complexation of Ni(II) and Zn(II) with Cl− ions has been studied by calorimetry and spectrophotometry in N,N-dimethylformamide (DMF) containing 0.4 mol dm−3 (C2H5)4NClO4 as a constant ionic medium at 25 °C. Calorimetric titration curves obtained in each system were well-explained in terms of formation of [MCln](2−n)+ (n=1–4 and M2+=Ni2+ or Zn2+), and their formation constants, enthalpies and entropies were determined. Formation of the nickel(II) chloro complexes in DMF was also supported by spectrophotometry. In the nickel(II) chloride system, the stepwise enthalpy ΔH3° and entropy ΔS3° values are remarkably large and positive as compared with the ΔHn° and ΔSn° (n=1, 2, and 4) values. Individual electronic spectra of nickel(II) ion, [NiCl]+ and [NiCl2] are significantly different in both intensities and peak positions from those of [NiCl3]− and [NiCl4]2− over the range 300–850 nm. These thermodynamic and spectrophotometric results were explained in terms of formation of the octahedral [NiCl]+ complex as well as [Ni(dmf)6]2+ and of formation of the tetrahedral [NiCl3]− and [NiCl4]2− complexes in DMF. On the other hand, in the zinc(II) chloride system, the ΔSn° (n=1 and 2) values are large and positive, but the ΔSn° (n=3 and 4) values are significantly small. The corresponding ΔHn° (n=1 and 2) values are positive but the ΔHn° (n=3 and 4) values are significantly negative. It is thus postulated that the [ZnCln](2−n)+ (n=2 and 3) complexes have the tetrahedral coordination structure as well as the [ZnCl4]2− complex in DMF.