Oxidation of first-row bivalent transition-metal complexes containing ethylenediaminetetra-acetate and nitrilotriacetate ligands by free radicals: a pulse-radiolysis study

Abstract

The kinetics of reaction of OH radicals with the ligands (L) ethylenediaminetetra-acetate (edta) and nitrilotriacetate (nta), and their complexes [ML](M = Zn^II, Cu^II, Ni^II, Co^II, Fe^II, and Mn^II), have been studied. All these reactions have similar rate constants, although a hydrogen-abstraction reaction from the ligand is observed for the free ligands and their complexes with Zn^II, Mn^II, and Co^II. For [Co(edta)]^2– a partial oxidation to [Co(edta)]^– is observed. On the other hand, when M = Ni^II, Cu^II, or Fe^II the metal is oxidized by the OH radicals. The reasons for these observations are discussed in detail. The spectra, and kinetics of decomposition, of the unstable intermediates formed by the reactions of the OH radicals are reported and discussed. The reactions of nickel(III) complexes with I^–, Br^–, and O₂ are reported. The kinetics of oxidation of the complexes (M = Fe^II, Mn^II, or Co^II) by [O₂]^–·, ˙O₂CH₂CMe₂OH, [Br₂]^–·, [(NCS)₂]^–·, and [I₂]^–·, the latter only with M = Fe^II, have also been studied. The kinetics indicate an inner-sphere mechanism for these reactions. The kinetics of hydrolysis of [Mn^IIIL(X)] and [Co^IIIL(X)] thus formed are reported.