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The study on the co-ordination chemistry of the complexes with thiolic heterocyclic ligands is based on the ion metals binding precisely to the sulphur atom. Studies were consequently carried out on the complexes with some thiolic ligands, such as: 2-imidazolidine-thione (IMT), hydantoin (ID), 2-thiohydantoin (TIOID), rhodanine (RD), 2-mercaptoimidazole (MI), 2-mercapto-1-methylimidazole (MMI) and 2-mercaptopyridine (MPYR), supposing that the co-ordination bond between sulphur and metal is stronger than the possible bond between nitrogen or oxygen and metal due to the minor difference in electronegativity existing between sulphur and metal compared with that existing between nitrogen or oxygen and metal. Complexes of the kind we prepared generally present relatively low constants of instability, marking a strong bond between the metal and the ligand. This would seem to be in direct correlation with the anti-bacterial and herbicidal activity. The activity of the complex in fact proves to be more marked than that of the pure ligand, and the increase appears to be in proportion to the strength of the bond between the metal and the ligand, as seen by the low values of the relative constants of instability. We may imagine that the decrease in the constants of instability corresponds to an increase in the strength of the co-ordination bond and to a parallel increase in the activity and the hypothesis is in accordance with the premised correlation between the strength of the metal-ligand bond and the biological activity.