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 [(CoIII)2]//dt=kCr[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 kcr[Cr2+]+k1. At 25 °C and µ= 1·0M(LiClO4), kcr=(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), kcr=(4·7 ± 0·5)× 10–4 l mol–1 s–1. Activation parameters have been obtained corresponding to kcr 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), kv=(6·0 ± 0·3)× 10–2 l mol–1 s–1, for the –d ln [(CoIII)2]/dt=kv[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 kcr : kv 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.