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| - Impact of Ligand Protonation on Higher-Order Metal Complexation Kinetics in AqueousSystems
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| - The impact of ligand protonation on the complexation kinetics of higher-order complexes is quantitativelydescribed. The theory is formulated on the basis of the usual situation for metal complex formation in aqueoussystems 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 MLn species that account forthe 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 MLn → 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 ofCd(II) with pyridine-2,6-dicarboxylic acid fall into this category, and their lability at a microelectrode isreasonably well predicted by the differentiated approach. For non-dynamic systems, the kinetic flux arisingfrom dissociation of higher-order complexes contributes to the rate-determining step. In this case, the weightedcontribution of protonated and unprotonated outer-sphere complexes in all contributing dissociation reactionsmust be taken into account. The kinetic flux arising from the dissociation of 1:2 complexes of Ni(II) withbicine at a conventional electrode was quite well described by this combined approach. The results establishthe generic role of ligand protonation within the overall framework of metal complexation kinetics in whichcomplexes may be dynamic to an extent that depends on the operational time scale of the measurementtechnique.
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