One-electron redox kinetics of aqueous transition metal couples Zn2+/+, Co2+/+, and Ni2+/+ using pulse radiolysis

Abstract

The one-electron redox potentials for aqueous metal couples Co^2+/+ and Ni^2+/+ have been investigated by using pulse radiolysis using their reactions with a series of reference compounds to establish the most positive upper limits of E⁰. Experiments with Zn⁺ were also carried out to confirm the characteristic shape of the expected reduction kinetics. Both formate ions and t-BuOH were employed to scavenge ˙OH radicals and ˙H atoms. Kinetics and fitted first and second order reaction rates have been reported for reactions with methyl viologen, fluorescein, Ru(NH₃)₆³⁺, Co(en)₃²⁺, Co(sepulcrate)³⁺, Ru(bpy)₃²⁺, Cr(bpy)₃³⁺, and Ni(Me₆[14]4,11-dieneN₄)²⁺. Previous work demonstrated that both Co²⁺ and Ni²⁺ can be reduced by CO₂˙⁻ radicals, giving a negative E⁰ limit of −1.9 V vs. SHE. A definite reaction of Ni⁺ with fluorescein di-anions provides a new upper limit of the Ni^2+/+ couple as −0.906 V vs. SHE. The reaction of Co⁺ with Ru(bpy)₃²⁺ has been confirmed, giving E⁰ = −1.3 V vs. SHE as a rigorous upper limit of the Co^2+/+ couple. In the case of Co^2+/+, kinetics were complicated by a self-catalyzed metal clustering phenomenon. Initiation rate constants of this process have also been reported.