Calorimetric and Raman Spectroscopic Studies on Formation of Cadmium(II) Thiocyanato Complexes in Aqueous Solution

Abstract

Abstract Complex formation equilibria between cadmium(II) and thiocyanate ions have been calorimetrically studied at 25 °C in aqueous solution containing NaClO4 in order to keep 3 mol dm−3 ionic strength in the test solutions. Calorimetric titration curves thus obtained were well explained in terms of formation of dinuclear [Cd2X]3+ (X=SCN) and mononuclear [CdXn](2−n)+ (n=1–4) complexes, and their formation constants, enthalpies and entropies were determined. The stepwise enthalpy and entropy values at the third consecutive step were largely positive, indicating that water molecules solvating Cd(II) ion were extensivelyl liberated at this step. It was thus suggested that the coordination symmetry around Cd(II) ion changed from octahedral to tetrahedral at the formation of [CdX3]−. Raman spectra of the C–S and C–N stretching vibrations of SCN− ion were measured in cadmium(II) thiocyanate solutions with varying molar ratios of the metal to ligand ions. It was shown that SCN− ion coordinated to Cd(II) ion through the N atom at the first step of the complex formation, and the S atom at the second, N at the third and then S at the last. The suggested bonding type of SCN− ion with Cd(II) ion within each cadmium(II) thiocyanato complex was supported by the thermodynamic data.