Abstract: The periodate complexing solution of the quadrivalent state praseodymium (Ⅳ) and terbium (Ⅳ) was prepared in alkaline solution by oxidizing the corresponding Ln (Ⅲ) periodate complexes with ozone.The chemical analysis results indicate that a 1 < 1(IO₆:Ln) type complexing anion can occur in the solution, in which almost 100% terbium and only 60% praseodymium can be stabilized in the + 4 state due to the presence of periodate ion H₂IO₆^(S-n)-, which is a polydentate ligand and often be used for stabilizing rather unusual and high oxidation state.The characteristic L→Melectron-transfer absorption bands of the Ln (Ⅳ) periodate complexes were observed in the UV-Visible region and well assigned, the wave number at maximum absorption for each of the Ln (Ⅳ) periodate complexes was measured with the corresponding molar absorptivity and the half-width at half-maximum absorption on the low wave number side δ(-).In addition, the spontaneous reduction of the Ln (W) complexes in the alkaline solution was studied by means of spectrophotometric method, all the kinetic parameters, including the observed rate constants K_oba.at different temperatures, the activatin energy E_m, the enthalpy and entropy of activation △H^≠, △S^≠, were obtained from the decay of the ab-sorbance with increasing the time.The results indicate that the reactins belong to the activation-controlled electron-transfer process, which were independent of the hydroxide ion and involved a free radical intermediate of periodae.

Key words: Perlodate complexes of lanthanides (Ⅵ), Electron-transfer reaction, Kinetics, Mechanism