One-electron redox kinetics of aqueous transition metal couples Zn 2+/+ , Co 2+/+ , and Ni 2+/+ using pulse radiolysis.

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 ofE(0). Experiments with Zn(+)were also carried out to confirm the characteristic shape of the expected reduction kinetics. Both formate ions andt-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)(6)(3+), Co(en)(3)(2+), Co(sepulcrate)(3+), Ru(bpy)(3)(2+), Cr(bpy)(3)(3+), and Ni(Me-6[14]4,11-dieneN(4))(2+). Previous work demonstrated that both Co(2+)and Ni(2+)can be reduced by CO(2)(-)radicals, giving a negativeE(0)limit of -1.9 Vvs.SHE. A definite reaction of Ni(+)with fluorescein di-anions provides a new upper limit of the Ni(2+/+)couple as -0.906 Vvs.SHE. The reaction of Co(+)with Ru(bpy)(3)(2+)has been confirmed, givingE(0)= -1.3 Vvs.SHE as a rigorous upper limit of the Co(2+/+)couple. In the case of Co2+/+, kinetics were complicated by a self-catalyzed metal clustering phenomenon. Initiation rate constants of this process have also been reported.