Synthesis, spectroscopic characterization, pH-metric and thermal behavior on Co(II) complexes formed with 4-(2-pyridyl)-3-thiosemicarbazide derivatives

Four new cobalt (II) complexes of some thiosemicarbazides have been synthesized and spectrochemically characterized. The thiosemicarbazides are prepared by the addition of 4-(2-pyridyl)-3-thiosemicarbazide to phenyl isothiocyanate (H2PPS), benzoyl isothiocyanate (H2PBO), phenyl isocyanate (H2APO) and 2-pyridyl isothiocyanate (H2PPY). The complexes are characterized by elemental analysis, spectral (IR, 1H NMR and UV–Vis), thermal and magnetic measurements. The studies revealed that structures of complexes are of two types octahedral and tetrahedral. The octahedral complexes are of H2PPS, which acts as di-anionic tetradentate SSNN and H2APO acts as mono-anionic tridentate NON. The tetrahedral complexes are of H2PBO, which acts as di-anionic tridentate NSO and H2PPY acts as mono-anionic bidentate NS. From the modelling studies, the bond length, bond angle, HOMO, LUMO and dipole moment had been calculated to confirm the geometry of the ligands and their investigated complexes. From TG and DTA studies kinetic parameters are determined using Coats–Redfern and Horowitz–Metzger methods. From pH metric studies at 298, 303 and 308 K and μ (0.1, 0.15 and 0.2) in 50% dioxane–water mixture the protonation constants of the ligands, the stepwise stability constants of the complexes and their thermodynamic parameters are calculated.