Cr2+ and V2+ reductions of the µ-amido-µ-formato-bis[tetra-amminecobalt(III)] and µ-amido-µ-acetato-bis[tetra-amminecobalt(III)] complexes

Abstract

The first and only observable stage in the Cr²⁺ reduction of µ-amido-µ-formato-bis[tetra-amminecobalt(III)] perchlorate(I), in aqueous perchlorate media, obeys the rate law (i), where the Cr²⁺ reactant is present in large excess. –d ln [(Co^III)₂]//_dt=k_Cr[Cr²⁺]+k₁k₂[Cr²⁺]//_{k–1+k2[Cr²⁺]}(i)

With [Cr²⁺] > ca. 0·015M, so that k₂[Cr²⁺]≫k_–1, the right-hand side of (i) reduces to k_cr[Cr²⁺]+k₁. At 25 °C and µ= 1·0M(LiClO₄), k_cr=(1·68 ± 0·02)× 10^–3 l mol^–1 s^–1 and k₁=(9·2 ± 0·8)× 10^–6 s^–1. For the µ-amido-µ-acetato-bis[tetra-amminecobalt(III)] complex, only the first term in (i) could be detected with certainty and at 25° and µ= 1·0M (LiClO₄), k_cr=(4·7 ± 0·5)× 10^–4 l mol^–1 s^–1. Activation parameters have been obtained corresponding to k_cr for both complexes and k₁ for the µ-formato-complex (I). Reduction of the complexes with V²⁺ follows the rate law (ii). At 25° and µ= 1·0M (LiClO₄), k_v=(6·0 ± 0·3)× 10^–2 l mol^–1 s^–1, for the –d ln [(Co^III)₂]/dt=k_v[V²⁺](ii)µ-formato-complex (I) and (2·2 ± 0·1)× 10^–2 l mol^–1 s^–1 for the µ-acetato-complex. The ratios k_cr : k_v are 0·028 and 0·021 respectively, suggesting that an outer-sphere mechanism is operative in each case. The effect of chloride ions on the Cr²⁺ reduction of the µ-formato-complex(I) has also been studied.