镧系元素的电子转移反应(Ⅰ)——Ln(Ⅳ)IO_6^-在KOH水溶液中还原反应的动力学和机理

Electron-Transfer Reaction of Lanthanides(Ⅰ) ——-Kinetics and Mechanism of the Spontaneous Reduction of Pr(Ⅳ),Tb(Ⅳ) Complexes with Periodate in KOH Solution

研究了Pr(Ⅳ)、Tb(Ⅳ)高碘酸根配合物的L→M电荷迁移跃迁,利用紫外-可见分光光度计探讨了配合物阴离子在KOH水溶液中自还原反应的动力学规律,测出了电子转移反应过程的动力学参数。根据实验现象及测定结果推出了Ln(Ⅳ)IO_6^-配离子在水溶液中自还原反应的机理,并对其进行了验证。

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 H2IO-(5-n)- , which is a polydentate ligand and often be used for stabilizing rather unusual and high oxidation state. The characteristic L→M electron-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 e and the half-width at half-maximum absorption on the low wave number side Ⅷ( - ). 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 too. at different temperatures, the activatin energy Em , 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.