The coordination of reduced species of oxygen to niobium and tungsten and the synthesis of peroxo complexes provide an insight on the redox reactions such as the oxidation of halides and inorganic and organic substrates or aromatic derivatives. Furthermore, the iron enzymes catalyze a variety of oxygen atom redox reactions. Determination of the structure of synthetic analogues gives information on the catalytic mechanism of the enzyme. The present thesis concerns the synthesis, characterization and study on the redox properties of niobium, tungsten and iron complexes with chelating agents that are of biological interest. The dihydroxy benzene and pyridine derivatives (2,3-DHBA, 3,4-DHBA, 2,5-DHBA, 2,3-DHP and quinoline-2-carboxylic acid) are multidentate ligands, important metabolites and antioxidants. Studying the activity of niobium complexes towards the activation of Ο2 included the reaction of NbCl5 in methanol media with the halides. The oxidation of PPH4Br in the presence of oxygen and light catalyzed by NbCl5 led to the crystal tribromide. The reaction of the excited catalyst with bromide ion occurs through the formation of the peroxoniobate intermediate. The role of niobium pentachloride in the cyclisation of acetone is also discussed. The triacetone triperoxide was characterized using X-ray diffraction spectroscopy. In order to compare the antioxidant activity of the ligands, ascorbic acid has been used as a reference antioxidant. Mixing methanol solutions of NbCl5 and AscA in the presence of oxygen and base gives upon their reaction the first peroxo complex of the metal with ascorbic acid isolated without adding hydrogen peroxide solution. The title compound shows an increased antiproliferative effect on HL60 human leukemia cells, compared to ascorbic acid alone, as well as an inhibitory effect on necrosis caused by ascorbic acid. The homoleptic peroxo complexes of Nb(V) and W(VI) have been prepared according the procedure based on the addition of excess of hydrogen peroxide in the aqueous solution of the precursors Nb(V) and W(VI). The heteroleptic peroxo complexes of Nb(V) and W(VI) with quinaldic acid of the formula (gu)2[Nb(O2)3(quin-2-c)] and (gu)[WO(O2)2(quin-2-c)] were determined by X-ray diffraction. All complexes were characterized by elemental and thermogravimetric analysis (TGA and DTA) and various spectroscopic techniques such as UV-Vis, IR, Raman and NMR. The ground electronic structure of the (gu)2[Nb(O2)3(quin- 2-c)] complex has been calculated with the density functional theory (DFT) method, and TD-DFT calculations have been employed to interpret the UV-Vis transitions. The in vitro insulin mimetic effect of the peroxo complexes has been evaluated by the inhibitory effect on free fatty acid release in isolated fat adipocytes treated with epinephrine. Among the iron complexes, we were concerned about the characterization and the redox properties of those with the ligands 2,3-DHBA and 3,4-DHBA. For this purpose gallium complexes homologues have been also synthesized. Their redox properties were studied through cyclic voltammetry. The results show that the semiquinonate-type of the ligand 2,3-dihydroxybenzoic acid (2,3-DHBA) is stabilized upon the coordination. The oxidative degradation of the aromatic ring of DHBAs in the presence of Nb(V) and W(VI) peroxo complexes and the analysis of the products, using NMR spectroscopy, suggest that oxygen activation occurs. The oxidative cleavage can take place either via semiquinone or directly via coordinated catechol.