Eigen-Wilkens mechanism

Detailed kinetic data are rare for natural aquatic ligands. For simple, not strongly binding ligands, it has been shown [164,165,171] that the dehydration of M(H20)q+ subsequent to the formation of an outer sphere complex or ion pair (Eigen-Wilkens mechanism, equation (27)) is often the rate-limiting step in the formation of the metal complex, ML. This mechanism has often been applied to natural ligands [5,167,171] without further confirmation of its validity. 

Application of the Eigen-Wilkens Mechanism to Natural Waters... 

The Eigens-Wilkens mechanism is not applicable for several environmentally relevant conditions [165,171] ... 

In summary, complexation kinetics for most 1 1 metal ligand complexes are generally rapid, with water-loss rate constants, k, ranging from 104 to 109 s-1 [164], In many cases involving the formation of simple divalent metal complexes, the Eigen Wilkens mechanism gives a reasonable estimate of a maximum value of k[. Nonetheless, it must be kept in mind that in complicated... 

In the simplest case, it can be assumed that the water exchange reaction with a complexing site of a heterogeneous ligand will be governed by the Eigen-Wilkens mechanism as for complexes with simple ligands. In this case, equations (26) and (29) can be combined to yield ... 

Figure 4. Copper complexation by a pond fulvic acid at pH 8 as a function of the logarithm of [Cu2+]. On the x-axis, complex stability constants and kinetic formation rate constants are given by assuming that the Eigen-Wilkens mechanism is valid at all [M]b/[L]t. The shaded zone represents the range of concentrations that are most often found in natural waters. The + represent experimental data for the complexation of Cu by a soil-derived fulvic acid at various metakligand ratios. An average line, based on equations (26) and (30) is employed to fit the experimental data. Data are from Shuman et al. [2,184]...

Times based on the assumption of an Eigen-Wilkens mechanism (see Section 4.3.1) for... 

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