Impact of Ligand Protonation on Higher-Order Metal Complexation Kinetics in Aqueous Systems

Abstract

The impact of ligand protonation on the complexation kinetics of higher-order complexes is quantitatively described. The theory is formulated on the basis of the usual situation for metal complex formation in aqueous systems in which the exchange of water for the ligand in the inner coordination sphere is rate-determining (Eigen mechanism). We derive expressions for the general case of lability of ML(n) species that account for the contributions from all outer-sphere complexes to the rate of complex formation. For dynamic complexes, dissociation of ML is usually the rate-determining step in the overall process ML(n) --> M. Under such conditions, it is the role of ligand protonation in the step ML --> M that is relevant for the kinetic flux. 1:2 complexes of Cd(II) with pyridine-2,6-dicarboxylic acid fall into this category, and their lability at a microelectrode is reasonably well predicted by the differentiated approach. For non-dynamic systems, the kinetic flux arising from dissociation of higher-order complexes contributes to the rate-determining step. In this case, the weighted contribution of protonated and unprotonated outer-sphere complexes in all contributing dissociation reactions must be taken into account. The kinetic flux arising from the dissociation of 1:2 complexes of Ni(II) with bicine at a conventional electrode was quite well described by this combined approach. The results establish the generic role of ligand protonation within the overall framework of metal complexation kinetics in which complexes may be dynamic to an extent that depends on the operational time scale of the measurement technique.

Contribution of Partially Labile Complexes to the DGT Metal Flux

“…A check of the accuracy of eqs and in reproducing the metal flux and the lability degree was performed by considering experimental results for the Cd-NTA system (Figure ). Due to proton and Cd competition in the binding to NTA, some transformations are required to convert the formulation of this system into one formally identical to only one complex being present, so that eq can be applied. , …”

Outer-Sphere and Inner-Sphere Ligand Protonation in Metal Complexation Kinetics: The Lability of EDTA Complexes

“…The first detailed treatment of the topic for 1:1 ML innersphere complexes (22) derived expressions for the lability of metal complexes with protonated and unprotonated ligand species being involved in formation of the precursor outersphere complex. This approach was then extended to the case of MLn complexes for which the dissociation to free M involves a sequence of dissociation steps (23). A proton anywhere on the ligand lowers the stability of the metal complex and affects the dissociation rate, irrespective of whether the pertaining site is involved directly in the metal complexation.…”

Electrochemical Methods for Speciation of Trace Elements in Marine Waters. Dynamic Aspects

“…The stability of the outer-sphere complex for multidentate ligands containing several protonated sites was thoroughly investigated. The ligand protonation impact on the complexation kinetics of higher-order complexes (ML 2 ···ML n ) was analyzed and expressions including contributions of all outer-sphere complexes to the rate of complex formation were derived . The effect of the ligand protonation was also investigated in the formation/dissociation kinetics of inner sphere metal complexes .…”

“…The ligand protonation impact on the complexation kinetics of higher-order complexes (ML 2 ···ML n ) was analyzed and expressions including contributions of all outer-sphere complexes to the rate of complex formation were derived. 18 The effect of the ligand protonation was also investigated in the formation/dissociation kinetics of inner sphere metal complexes. 19 Although all protonated forms of the ligand contribute to the formation of the precursor outersphere complexes, only the sufficiently stable ones effectively contribute to the overall rate of inner-sphere complex formation, even if they are minor components in bulk solution.…”