Polarography of Metal Complexes in Non-aqueous Solvents

Abstract

Polarograhic studies of metal complexes in non-aquoes solvents have been reviewed from the following four standpoints; 1) Unusual oxidation states of metal complexes, 2) Effects of cations on the polarographic behavior of metal complexes, 3) Dissociation reaction of metal complexes, and 4) Analytical applications. 1) Unusual oxidation states of metal complexes. The use of aprotic solvents may give very interesting results in the study of electron transfer reactions of complexes, because some complexes of unsual oxidation states are stabilized considerably. Tris dipyridil complex of iron (Fe(dipy)₃²⁺) in aqueous alkaline medium shows a single polarographic wave of two-electron reduction process end reduced to metallic iron. but in acetonitrile, it gives three succesive one-electron reduction waves which represent the following process. Fe(dipy)₃²⁺ e ⇔Fe(dipy)₃¹⁺⇔Fe(dipy)₀ e ⇔Fe(dipy)₃^-1 Other dipyridil complexes of the first transition elements, such as Cr, Mn, Go, and Ni, have been found to give stepwise reduction waves in acetonitrile resulting in an anionic complex at the final step. Many electron transfer series have been observed for the complexes with delocalized ground states. With o-phenylendiamine compounds of Ni²⁺, Pd²⁺, and Pt⁺², four one-electron reactions have been found to occur following the process; M(o-Phen)₂²⁺ e ⇔M(o-Phen)₂¹⁺ e ⇔M(o-Phen)₂⁰ e ⇔M(o-Phen)₂^-1 e ⇔M(o-Phen)₂^-2. Here o-Phen is o-phenylendiamine ligand. In these complexes, a small change in the chemical structure of the ligand would be reflected in a shift of the half-wave potentials. Polarographic half-wave potentials for the follwoing two electrode reactions of metal dithiodiketonates; M(S₂C₂R₂)_n+e⇔M(S₂C₂R₂)_n^-1+e⇔M(S₂C₂R₂)_n^-2, have been reported for various substituents R. The E½ values for both processes have been found to have linear relationships with the inductive substituent constants. Cis and trans Ni(II) compounds with cyclic amine ligands, such as 14-diene and 14-tetramine, give one-electron reduction waves at a dropping mercury electrode and one-electron oxidation waves at a platinum electrode in acetonitrile. Those waves represent the reduction of NiL²⁺ to NiL⁺ and the oxidation of NiL²⁺ to NiL³⁺, respectively. The complexes of NiL³⁺ and NiL⁺ can be generated by controlled potential electrolysis and successfully isolated. They show paramagnetic properties which are due to d7 and d9 structure of Ni. From these, the electronic change caused by the electrode reaction seems to occur at the central atoms of the compounds. From the polarographic informations, only the total number of redox equivalent consumed at the electrode reaction can be determined. It is necessary to define the orbital into which the electron is accepted or from which the electron is removed. ESR spectrum affords a useful information on the localization of an unpaired electron. Holm et al. have investigated ESR spectra of monoanions of [M-S₄] or [M-N₂S₂] type metal complexes and estimated the orbital characters which participate the electronic change in the electron transfer processes by comparing g value of complexes with that of free electron. 2) Effects of some cations on the polaro ggraphic behavior of metal complexes. The polarographic waves of metal chelates have often been found to be affected by the reaction of anionic reduction products with metal ions.