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, 1:1 ) with thiolsalicylic acid (TSA) has been investigated spectrophotomet rically in ethanol-water solution (50%, v/v) at I = 0.1M (NaClO4) and 25-degrees-C. Under the acidic conditions encountered in this study, t here are competing equilibria between protonation of the Cu2+-MENA bin ary chelate, the formation of the Cu(MENA)(TSA) ternary complex and pr otonation of the second ligand (TSA). The equilibrium constants for th e complexation reactions and the stability of the mixed-ligand complex are determined. The optimum conditions for the predominance Of the te rnary complex are established and the enhancement of this complex over binary complex formation is evaluated. Complex-forming equilibria hav e also been examined by potentiometric-pH titrations and the experimen tal data are discussed in relation to various equilibria existing in s olution. Structural and bonding features of the mixed-ligand complex a re illustrated from considerations of the IR spectral data.