Thermodynamics of complexation of isatin-3-thiosemicarbazone (HIT) and other related derivatives with some metal ions

Proton-ligand formation constants of isatin-3-thiosemicarbazone (HIT) ; N-acetylisatin-3-thiosemicarbazone (HA1T) and 5-(p-nitrobenzoyl)-1,2,4-triazino[5,6-b]indole-3-thione (HBITr) ligands and their corresponding metal-ligand formation constants with Mn 2 + , Fe 3 + , Co 2 + , Ni 2 + , Cu 2 + , Zn 2 + , Cd 2 + , UO 2 2+ and Th 4 + ions were determined pH-metrically at 10, 20, 30 and 40 °C in 75 %(v/v) ethanol water. The thermodynamic parameters (ΔG, AH and AS) were also evaluated. It was found that both log K 1 and ΔH 1 , for HIT and HAIT-complexes are somewhat larger than log K 2 and ΔΗ 2 , indicating a change in the dentate character of these ligands from tridentate (ONN-donors) in 1:1 chelates to bidentate (ON-donors) in 1:2; M:L chelates. In contrast, the values of log K 2 and -ΔH 2 for HBITr-complexes are somewhat larger than log K 1 and -ΔH 1 , indicating a strong trans-effect for the second coordination. The dissociation process is non-spontaneous, endothermic and entropically unfavourable while the complexation process is spontaneous, exothermic and entropically favourable. The thermodynamic parameters were separated into their electrostatic (cl) and non-electrostatic (non) constituents.