Solution equilibria of pyridinecarboxylic acids: Complexation reaction of 2-mercaptonicotinic acid-copper (II) complex with thiosalicylic acid

The reaction of 2-mercaptonicotinic-copper (II) complex (Cu²⁺-MENA, 1 3n 1) with thiosalicylic acid (TSA) has been investigated spectrophotometrically in ethanol-water solution (50%, v/v) at I=O.IM (NaClO₄) and 25°C. Under the acidic conditions encountered in this study, there are competing equilibria between protonation of the Cu²⁺-MENA binary chelate, the formation of the Cu(MENA)(TSA) ternary complex and protonation of the second ligand (TSA). The equilibrium constants for the complexation reactions and the stability of the mixed-ligand complex are determined. The optimum conditions for the predominance of the ternary complex are established and the enhancement of this complex over binary complex formation is evaluated. Complex-forming equilibria have also been examined by potentiometric-pH titrations and the experimental data are discussed in relation to various equilibria existing in solution. Structural and bonding features of the mixed-ligand complex are illustrated from considerations of the IR spectral data.