Oxidation of Chromium(II) Complexes. I. Kinetics of the Oxidation of Tris(2,2′-bipyridine)- and of Tris(1,10-phenanthroline)chromium(II) Complexes with Hexaamminecobalt(III) Ion

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Kinetic studies have been carried out on the oxidation of [Cr(bipy)3]2+ and [Cr(phen)3]2+ with [Co(NH3)6]3+(bipy and phen denote 2,2′-bipyridine and 1,10-phenanthroline, respectively). The rate law is found to be −d[Cr(II)]/dt=k1[Co(III)][Cr(II)]+k2[Cr(II)]. Simulation procedures with an analogue computer were applied for the analysis of these reactions. The rate constant k1 (direct oxidation) and k2 (dissociation) for [Cr(bipy)3]2+ were 180 M−1 s−1 and 0.120 s−1 (μ=0.11 with NaCl, at 25 °C) and those for [Cr(phen)3]2+ were 310 M−1 s−1 and 0.630 s−1 (μ=0.20 with NaCl, at 25 °C), respectively. The larger value of k1 for [Cr(phen)3]2+ as compared with that of k1 for [Cr(bipy)3]2+ was interpreted as being due to the good electron conductivity of the phenanthroline as a ligand in the activated state. The activation parameters were obtained as follows: in the case of [Cr(bipy)3]2+, ΔH\neweq=7.5 kcal/mol and ΔS\neweq=−23 e.u. for the k1 path, and ΔH\neweq=17.2 kcal/mol and ΔS\neweq=−5 e.u. for the k2, path; and in the case of [Cr(phen)3]2+, ΔH\neweq=8.2 kcal/mol and ΔS\neweq=−20 e.u. for the k1 path, and ΔH\neweq=21.2 kcal/mol and ΔS\neweq=+12 e.u. for the k2 path.
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