The Cr(II) reduction of pentane-2,4-dionatobis(ethylenediamine)cobalt(III). An apparent failure of the product criterion for assigning inner-sphere mechanisms

The Cr(II) reduction of [Co(bipy)₂(ptdn)²⁺ (bipy = 2,2t́-bipyridine and ptdn = pentane-2,4-dione) proceeded too rapidly to be detected by the stopped-flow method and a lower limit of 3 × 10⁴ M^-1 s^-1 (μ = 0.10 M (LiClO₄)) and [H⁺] = 0.030 M was calculated for this reaction. Product studies established that both [Cr(bipy)(OH₂)₄]³⁺ and [Cr(bipy)(ptdn)(OH₂)₂]²⁺ were formed. The ration of products obtained was strongly dependent on the mixing technique used. Under conditions of excess chromium(II), more than the stoichiometric amount of Cr(III) products were isolated, indicating that some degree of substitution was occurring. Blank experiments established that the products obtained could have arisen from an outer-sphere reaction followed by capture of the free ligands by Cr(II). A synergistic effect between the acac and bipy ligands appears to be operating. Two reactions were observed on the stopped-flow instrument. Both are ascribed to ring-closure of bipy on Cr(III). The reaction to give [Cr(bipy)(OH₂)₄]³⁺ was characterised by k^{25 °C} = 5.06 × 10^-1 s^-1 with activation parameters ΔH^≠ = 9.9 ± 0.3 kcal/mol, and ΔS^≠ = -26.8 ± 0.9 e.u. The second reaction was ascribed to the formation of [Cr(bipy)(ptdn)(OH₂)₂]²⁺ from [Cr(bipy)(ptdn)(OH₂)₃]²⁺ and had k^{25 °C} = 5.83 × 10^-2 s^-2; ΔH^≠ = 4.93 ± 0.06 kcal/mol, ΔS^≠ = 47.6 ± 0.2 e.u. The final reaction was attributed to breakdown of the Cr(III) products. It occured very slowly with k^{25 °C} = 4.6 × 10^-6 s^-1 (all values quoted at μ = 0.10 M (LiClO₄) and [H⁺] = 0.030 M).