Kinetics of the Reduction of the Linkage Isomers of the Pentaamminecobalt(III) Complexes of 4-Cyanobenzoic Acid by Chromium(II)

Robert J. Balahura, W. C. Kupferschmid, W. L. Purcell

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The chromium(II) reduction of the nitrile-bonded pentaamminecobalt(III) complexes with 3- and 4-cyanobenzoic acids has been studied. For the 4-cyanobenzoic acid complex, the rate law at 25°C and I = 1.0 mol L-1 was kobsd = (kH[H+] + kKa)/(Ka + [H+]). A nonlinear least-squares fit of the data to this equation gave kH = 0.290 ± 0.005 L mol-1 s-1, k = 0.99 ± 0.12 L mol-1 s-1, and Ka = 0.0031 ± 0.0007 mol L-1. The reduction was shown to occur via remote attack of Cr(II) to produce a (carboxylato)chromium(III) product. At higher temperatures the inverse hydrogen ion term disappears, and the activation parameters for the kH term were ΔH = 7.1 ± 0.5 kcal mol-1 and ΔS = -37 ± 3 eu. The V(II) reduction of this complex followed the rate law kobsd = k1 + k2[H+]-1 with k1 = 11.1 ± 0.2 L mol-1 s-1 and k2 = 0.21 ± 0.04 s-1 at 25°C and I = 1.0 mol L-1. The nitrile-bonded 3-cyanobenzoic acid complex is reduced by chromium(II) without ligand transfer and follows the simple rate law -d In [Co(III)]/dt = k2[Cr(II)]. At 25°C and I = 1.0 mol L-1, k2 = (4.89 ± 0.23) × 10-2 L mol-1 s-1, ΔH = 8.3 ± 0.4 kcal mol-1, and ΔS = -37 ± 2 eu. The reductions are discussed in terms of the reducibilities of the ligands and the stabilities of possible precursor complexes.

Original languageEnglish (US)
Pages (from-to)1456-1461
Number of pages6
JournalInorganic Chemistry
Issue number10
StatePublished - May 1983


ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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