Abstract
The kinetics of the oxidation of Co
II
L
n
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
II
L
n
] ≫ [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
)
i
-
d
Cr
VI
/
dt
=
k
2
Co
II
L
n
+ k
3
Co
II
L
n
2
HCrO
4
-
Both
k
2
and
k
3
showed acid-dependent and acid-independent pathways. The direct conversion Co
II
L
n
to Co
III
L
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
II
L
n
], 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
.