KEYWORDSDFT calculations have been conducted to understand the conformational and tautomeric preference of 4-amino-5-(2-hydroyphenyl)-2,4-dihydro-3H-1,2,4-triazole-3-thione ligand. The results suggest that the thione tautomer is found to be the most stable form in the gas phase, which is in very good agreement with the FT-IR and crystallographic data. The protonation constants of the ligand and the stability constants of its complexes with Mn(II), Co(II), Ni(II) and Cu(II) have been determined by potentiometry in water-ethanol (90:10, v:v) mixture. The absolute pKa values of the ligand have been calculated using DFT method to complement and help the assignment of potentiometric data. DFT calculations have also been carried out for some metallic complexes species formed in solution in order to propose their most probable structures.Potentiometry DFT calculations Stability constants 1,2,4-Triazole ligand Dissociation constants Thione-thiol tautomerism

The synthesis of novel heterocyclic [1,2,4]triazolo[3,4-b][1,3,4]thiadiazine hybrids by a bimolecular reaction of 2-(4-amino-5-mercapto-4H-[1,2,4]triazol-3-yl)phenol with an aromatic or heterocyclic α-bromoacetyl derivatives is described. This synthetic procedure starts from an unprotected phenol.