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 kobs=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 kcr/ky, ratios. Separation and deter-mination of the CrIII 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.