The Cr2+ and V2+ reduction of µ-carboxylato dicobalt(III) ammine com-plexes. Part VII. The preparation and mechanism of reduction of µ-malonato-, µ-dimethylmalonato-, and µ-glycolato-complexes

Abstract

Perchlorate saits of the µ-malonato-, µ-dimethylmalonato-, and µ-glycolato-complexes [(NH₃)₄Co·µ(NH₂,O₂CR)· Co(NH₃)₄]⁴⁺[R = CH₂CO₂H (I) and R = C(CH₃)₂CO₂H (Il)] and [(NH₂)₃Co·µ(OH,OH,O₃CR)·CO(NH₂)₃]³⁺[A = CH₂CO₂H (III), R = C(CH₃)₂CO₂H (IV), and R = CH₂OH (V)] have been prepared. Kinetic studies on the Cr²⁺ reductions of complexes (I)–(V), and the V²⁺ reductions of (I), (III), and (V) at 25 ″C, I= 1.00M(LiClO₄) are reported. Reduction of the first cobalt(III) is in all cases slow and rate determining. Onvarying thehydrogen-ion concentration from 0.10 to 0.90M, rate constants for complexes (I)–(IV) give a dependence k_obs=k₁+k₂[H⁺]^–1, where k₂ makes relatively large contributions for the µ-amido-complexes (I) and (II). No [H⁺]-dependence is observed for (V). The k₁, pathway is common to and about the same, (1–3.6)× 10^–3 I mol^–1 s^–1, for all Cr²⁺ reductions. Outer-sphere mechanisms are assigned to k₁ on the basis of k_cr/k_y, ratios. Separation and deter-mination of the Cr^III products, [Cr(H₂O)₆]³⁺(50%). [Cr(H₂O)₅(O₂CCH₂CO₂H)]²⁺(31%), and [Cr(H₂O)₄(O₂CCH₂CO₂)]⁺(19%), for the Cr²⁺ reduction of complex (I), [H⁺]= 0.05M. suggest that the Cr²⁺ attaches itself to the unprotonated (pendant) carboxylate for pathway k₂. Since the C-atoms of the malonate are saturated it is proposed that electron transfer from the attached reductant occurs as in an outer-sphere process and not via the malonate.