Outer-sphere electron transfer reactions of ...

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Title Outer-sphere electron transfer reactions of aqua(5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-1-aceta to) nickel(II) and (III).
Author(s) Robert I. Haines, D. R. Hutchings
Journal Canadian Journal of Chemistry-Revue Canadienne De Chimie
Date 2003
Volume 81
Issue 2
Start page 186
End page 192
Abstract The outer-sphere oxidation of the nickel(II) complex of the deprotonated pendant-arm macrocycle, 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-1-acetate, [NiL1(OH2)](+) by bis-(1,4,7-triazacyclononane)nickel(III), [Ni(tacn)(2)](3+) has been studied in aqueous perchlorate media. The reaction displays reversible second-order behaviour and the kinetic study reveals the forward and reverse rate constants for the reaction: [Ni(tacn)(2)](3+) + [NiL1(OH2)](+) reversible arrow [Ni(tacn)(2)](2+) + [NiL1(OH2)](2+) The kinetics show the forward reaction to be acid dependent, a feature that is attributed to protonation of the acetato group of the nickel(II) complex. Using Marcus theory, the self-exchange rate for the [NiL1(OH2)](+/2+) couple has been calculated. The nickel(II/III) electron transfer is a reversible one electron process with Edegrees = 1.04 V (vs. S.H.E.). The formation of the authentic nickel(III) product has been confirmed by esr spectroscopy. The kinetics of reduction of the [NiL1(OH2)](2+) species by Fe2+(aq) exhibits a second-order rate law, the reaction being independent of acid. Using the calculated self-exchange rate for the nickel complex, its reaction with Fe2+(aq) has been examined in terms of an inner- versus outer-sphere mechanism.

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