One- and two-electron reduction of chromium(VI) by polyaminocarboxylatocobaltate(II) complexes and the formation of chromium(V) and chromium(IV)

Abstract

The kinetics of the oxidation of Co^IILⁿ complexes {where L = ethylenediaminetetraacetate (EDTA), diethylenetriaminepentaacetate (DTPA), or N-(2-hydroxyethyl)ethylenediaminetriacetate (HEDTA)} by Cr^VI were studied under pseudo-first-order conditions with [Co^IILⁿ] ≫ [Cr^VI]. The kinetics showed first-order dependence on [Cr^VI]. The rate constant, k _obs, decreases with increasing concentration of [Cr^VI]. At constant [H⁺], ionic strength, and temperature, the rate law is described by Eq. (i)

$$ - {\text{d}}\left[ {{\text{Cr}}^{\text{VI}} } \right] / {\text{dt}} = \left\{ {{\text{k}}_{ 2} \left[ {{\text{Co}}^{\text{II}} {\text{L}}^{\text{n}} } \right]{\text{ + k}}_{ 3} \left[ {{\text{Co}}^{\text{II}} {\text{L}}^{\text{n}} } \right]^{ 2} } \right\}\left[ {{\text{HCrO}}_{4}^{ - } } \right] $$

(i)

Both k ₂ and k ₃ showed acid-dependent and acid-independent pathways. The direct conversion Co^IILⁿ to Co^IIIL^m is ruled out by spectrophotometric and ESR spectroscopic measurements that showed the formation of initial reaction intermediate(s). The rate law is consistent with one-electron and concurrent two-electron transfers leading to the formation of Cr^V and Cr^IV, respectively. An inner-sphere process, at least for the first term, leading to the formation of a relatively stable Cr^V species is almost certain. The kinetic term showing second-order dependence on [Co^IILⁿ], most likely, involves concurrent two-electron transfer leading to the formation of Cr^IV. The type of rate law and the proposed mechanism, reported here, depart from the well-established rate laws observed and mechanisms proposed for the oxidation of one-electron reductants by Cr^VI.