Vol. 7, Issue 11, Part A (2025)

The stability constants of complexes formed between some substituted pyrazolines and transition/lanthanide metal ions have been determined spectrophotometrically in 70% (v/v) ethanol-water medium at room temperature. The ligands studied include L4 and L6, representing differently substituted pyrazolines possessing potential donor sites such as azomethine nitrogen and carbonyl oxygen atoms. The metal ions investigated were Co(II) and Ni(II) with ligand L4, and La(III) and Tb(III) with ligand L6.The absorbance data at varying metal-to-ligand ratios were analysed using the Job’s method of continuous variation and mole-ratio method, from which the composition and formation constants (log K) of the complexes were evaluated. The results revealed the formation of 1:1 and 1:2 (metal: ligand) complexes depending on the system. The calculated stability constants indicated that all metal-ligand complexes are stable, with the order of stability following the Irving-Williams sequence for the transition metal complexes, i.e., Co(II) < Ni(II), and La(III) < Tb(III) for the lanthanide complexes. The relatively higher stability of the Tb(III) and Ni(II) complexes suggests stronger coordination through nitrogen and oxygen donor atoms, confirming the chelating ability of substituted pyrazolines. The study provides valuable insight into the coordination behaviour of substituted pyrazolines toward both transition and rare-earth metal ions and supports their potential application in analytical, catalytic, and biological systems.