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

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 k_obsd = (k_H[H⁺] + kK_a)/(K_a + [H⁺]). A nonlinear least-squares fit of the data to this equation gave k_H = 0.290 ± 0.005 L mol^-1 s^-1, k = 0.99 ± 0.12 L mol^-1 s^-1, and K_a = 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 k_H 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 k_obsd = 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.