The preparation and characterization of the µ-amido-µ-oxalato-bis[tetra-amminecobalt(III)] complex, and the kinetics of reduction with Cr2+ and V2+

Abstract

The chloride salt of the protonated form of the µ-amido-µ-oxalato-bis[tetra-amminecobalt(III)] complex, has been prepared and characterized. The perchlorate salt has also been prepared and it would appear that the oxalate [graphic omitted] bridge is only 50% protonated in this solid. From spectrophotometric measurements on the complex in acidic solutions at 15–45°, µ= 2·0M(LiClO₄), values of the protonation constant K= 18 ± 2 l mol^–1 have been determined. Reduction with chromium(II), 2Cr²⁺+(Co^III)₂→ 2Co²⁺+(Cr^III)₂, proceeds in two stages, each involving attack of the chromium(II) and reduction of cobalt(III). Both protonated (k_a) and unprotonated (k_b) forms participate in the first stage of the reaction (k₁), and the rate law (Cr²⁺ in large excess) is as in (i) [graphic omitted]. At 25° and µ= 2·0M(LiClO₄), k_a= 1·45 ± 0·07 l mol^–1 s^–1, and k_b= 35·2 ± 0·4 l mol^–1 s^–1. The second stage of the reduction is independent of [H⁺] as in (ii) [graphic omitted] and k₂= 1·88 ± 0·05 l mol^–1 s^–1 at 25°. Information regarding the spectrum of the intermediate and final product has been obtained. The reduction with V²⁺ also proceeds in two stages, the first of which is directly measurable. The rate law observed is of the same form as (i), and again both the protonated (k_c) and unprotonated (k_d) forms are reactive. At 25° and µ= 2·0M(LiClO₄), k_c= 3·51 ± 0·04 l mol^–1 s^–1 and k_d= 22·7 ± 0·4 l mol^–1 s^–1. Activation parameters have been obtained for k_a, k_b, k₂, k_c, and k_d.