Reactions of chromium(III) complexes of 1,10-phenanthroline, 2,2′-bipyridyl, and oxalate with the pulse radiolytically generated aquated electron, zinc(I), and cadmium(I)

Abstract

The chromium(III) ions [Cr(phen)₃]³⁺, [Cr(bipy)₃]³⁺, [Cr(phen)₂(ox)]⁺, [Cr(bipy)₂(ox)]⁺, [Cr(phen)(ox)₂]^–, and [Cr(bipy)(ox)₂]^–(phen = 1,10-phenanthroline, bipy = 2,2′-bipyridyl, ox = oxalate) react rapidly with the aquated electron to form the chromium(II) analogues. Rate constants for formation of the chromium(II) species are dependent on complex charge, increasing with increasing positive charge, but all greater than 10¹⁰ dm³ mol^–1 s^–1. Electronic spectra of the chromium(II) complexes prior to any dissociation were measured. While [Cr(phen)₃]²⁺ and [Cr(bipy)₃]²⁺ decompose only slowly in aqueous solution, all complexes with oxalate ligands decompose rapidly with first-order rate constants >8 × 10³ s^–1. This behaviour parallels the voltammetric behaviour at a glassy carbon electrode, where only [Cr(phen)₃]³⁺ and [Cr(bipy)₃]³⁺ exhibit any reversible character for the Cr^III–Cr^II couple. Electron-transfer reactions between the chromium(III) complexes and zinc(I) or cadmium(I), initiated radiolytically, have rate constants near 2 × 10⁹ dm³ mol^–1 s^–1 in all cases, and are probably diffusion controlled.