Potentiometric Studies on Binary and Ternary Complexes of Ni(II) and Cu(II) Ions with L-Valine and Paracetamol

The protonation constants of the free ligands and the stability constants of binary and ternary complexes of bivalent metal ions of Ni(II) and Cu(II) with a biologically important amino acid of L-valine, Val, and paracetamol, Para, were studied potentiometrically in aqueous solutions at 313.15 ± 0.1 K and a fixed ionic strength of I = 0.10 M NaCl. The complexation model for each system was established using the Irving-Rossotti equation. The formation of the 1:2 or 1:1 binary complexes and 1:1:1 ternary complexes in which the amino acid Val was used as the primary ligand and Para as the secondary ligand, as inferred from the corresponding potentiometric pH-metric titration curves, and their relative stabilities compared to the corresponding ML and ML2 binary complexes are expressed in terms of statistical parameters ∆logK, logK1 and logK2. The complex stability was found to follow the order of Cu(II) > Ni(II). Through these diagnostic studies, it was possible to give the general formula of compounds prepared from amino acids and paracetamol. Amino acid binds to the central ion through oxygen in the hydroxyl group and nitrogen atom in the amine group (-NH2), whereas paracetamol forms a unipolar bond by binding to the concentrated ion through the oxygen atom in the hydroxyl group. Most of the nickel complexes had octahedral symmetry with valine and paracetamol ligands, while the copper complexes had square or hierarchical to square base symmetry.