Spectroscopic and Calorimetric Investigations of Transition Metal Complexes with Schiff Base Ligands: Equilibrium, Stability, and Thermodynamic Perspectives

Abstract

The present investigation reports a systematic and comprehensive study of coordination behaviour between selected transition metal ions — specifically Cu(II), Ni(II), Co(II), Zn(II), and Mn(II) — and a novel tridentate Schiff base ligand, 2-hydroxy-1-naphthaldehyde-4-aminoantipyrine (HNAA), in a 70% dioxane-water binary solvent system at 298 K and ionic strength I = 0.1 M NaClO₄. The complexation equilibria were probed using UV-Vis electronic spectroscopy, potentiometric pH titration, and isothermal calorimetry to extract thermodynamic quantities including stability constants (log K), Gibbs free energy (ΔG°), standard enthalpy (ΔH°), and entropy changes (ΔS°). Spectral analysis confirmed octahedral geometry for Cu(II) and Ni(II) complexes, and tetrahedral geometry for Zn(II). The order of stability constants followed the Irving-Williams series: Cu(II) > Ni(II) > Co(II) > Zn(II) > Mn(II). Negative ΔG° values across all systems confirm the spontaneous nature of complex formation, while positive ΔS° values suggest an entropy-driven coordination process attributable to desolvation effects. The results provide mechanistic insight into the thermodynamic forces governing metal-ligand recognition and have implications for the design of metal-based pharmaceutical agents and catalytic systems.