Kinetics and mechanism of electron-transfer reactions of bis(1,4,7-triazacyclononane)nickel-(II) and -(III) complexes

Abstract

The rates of the outer-sphere electron-transfer reactions involving the complex ion [NiL¹₂]²⁺(L¹= 1,4,7-triazacyclononane) with a series of substituted polypyridine iron(III) ions, [Fe(L–L)₃]³⁺, and various macrocyclic nickel(III) species and of the reduction of [NiL¹₂]³⁺ by [Co(L–L)₃]²⁺ species have been measured. Use of a Marcus cross-correlation leads to a self-exchange rate for the [NiL¹₂]^3+/2+ couple of k₂₂= 6 × 10³ dm³ mol^–1 s^–1. The data are of interest in that, unlike most other Ni^II–Ni^III couples measured, upon electron transfer there is retention of octahedral symmetry around the metal centre. Possible correlations of rates of electron exchange with bond extension in the primary co-ordination sphere are discussed.