Eigen-Wilkins

These general comments regarding iron sequestration kinetics can be placed in the context of the Eigen-Wilkins mechanism for complexation (135). Initial complex formation proceeds by way of the formation of an encounter complex, Fe(H20)g+ L,... 

Several comprehensive texts [164-166] and papers [167-170] have been published on complexation reaction kinetics in aqueous, including environmental, solutions. In this section, we shall briefly examine the Eigen-Wilkins mechanism as a starting point for estimating the rates of metal complexation reactions in environmental aqueous systems (Sections 4.3.2-4.3.3) and as a basis for the definition of the lability criteria (Section 7.2). 

In the case of trace metals, adsorption is typically much faster than the time intervals for which it is practically possible to separate the cells. Therefore, in practice, values of kf and kr are most often estimated by assuming that water loss from the hydrated cation is rate-limiting (Eigen-Wilkins mechanism, see Section 4.3.1 above). In some cases, uptake transients can be observed at the start of a short-term uptake experiment or by using pulse-chase experiments for which a metal solution containing a radioactive tracer is replaced by a solution... 

Table IV is an attempt to summarize the results of these proton transfer studies in nonaqueous solvents. There is no systematic trend in what seems to be the rate limiting step in contrast to the attractive Eigen-Wilkins generalization for the mechanism of metal ion complexation. Obviously, many more proton transfer kinetic studies in nonaqueous solutions are needed for beautiful generalizations to emerge. Whether investigators will have the patience to carry them out or not is the only uncertainty.

As we have seen, an area of major importance and of considerable interest is that of substitution reactions of metal complexes in aqueous, nonaqueous and organized assemblies (particularly micellar systems). The accumulation of a great deal of data on substitution in nickel(II) and cobalt(II) in solution (9) has failed to shake the dissociative mechanism for substitution and for these the statement "The mechanisms of formation reactions of solvated metal cations have also been settled, the majority taking place by the Eigen-Wilkins interchange mechanism or by understandable variants of it" (10) seems appropriate. Required, however, are more data for substitution in the other... 

Eigen-Wilkins mechanism, 42 5 Einsteinium, 20 111 melting point, 31 6... 

A reaction sequence somewhat in parallel with the Eigen-Wilkins-Werner mechanism can also be expressed for the inner-sphere surface complexation of bivalent metal cations by an ionised surface hydroxyl group (cf equation (9.9a) ... 

A reaction rate law for the Eigen-Wilkins-Werner mechanism is developed in Section 1.5 (Eqs. 1.50, 1.52, 1.54a, 1.54c). If inner-sphere complex formation is rate limiting and the concentration of water remains constant, the rate of inner-sphere complex formation is (cf. Eq. 1,57)... 

In analogy with Eqs. 1.50 and 2.5, the overall surface ligand-exchange reactions in Eqs. 3.46, 3.53, 3.56, 3.65, 3.66, and 4.15b can be dissected into steps by applying the concept of the Eigen-Wilkins-Wemer mechanism, discussed in Section 2.1. Following this perspective, one would decompose the overall surface complexation reaction in Eq. 4.15b into a set of coupled reactions (cf. Eq. 1.50) ... 

The formation of copper(II) complexes with terpy has been investigated fairly intensively. The interaction is pH dependent, and numerous hydroxy, aqua, and polynuclear species are present in aqueous solution 94, 245,278). In general, an Eigen-Wilkins mechanism appears to be operative, although the kinetics are complicated by ligand-protonation equilibria 263,390,391). In acidic solution, 1 1 complexes predominate (567). A number of substituted terpyridine ligands have been evaluated as potential colorimetric reagents for copper 400). The adsorption behavior of copper(II)-terpy complexes at silica surfaces has been studied 499). Such complexes are reasonably active as catalysts for the hydrolysis of fluorophosphate esters 456). 

S213 As discussed in Section 21.6, the Eigen-Wilkins mechanism for substitution in octahedral complexes suggests the formation of an encounter complex [V(H20)6] Cl , in the first, fast step ... 

The second-order term in the rate laws for reactions of low-spin iron(II) diimine complexes with such nucleophiles as hydroxide and cyanide ions has been established as arising from a bimolecular reaction between complex and nucleophile.182 Activation volumes that were obtained for reactions of CN and OH with Fc(phcn)2 1 and Fe(bpy)3 + were in the range of +19.7 to +21.5cm3mol-1.183 Because these observations were not readily accounted for by an associative mechanism, a mechanism analogous to the Eigen-Wilkins mechanism of complex formation was introduced in which dissociative activation dominates in determining the observed activation volumes. However, subsequently it was shown that solvation... 

These apparent contradictions are explained by the Eigen-Wilkins mechanism. 

The Eigen- Wilkins mechanism applies to ligand substitution in an octahedral complex. An encounter complex is first formed between substrate and entering ligand in a preequilibrium step, and this is followed by loss of the leaving ligand in the rate-determining step. 

Consider reaction 25.22. The first step in the Eigen-Wilkins mechanism is the diffusing together of MLg and Y to form a weakly bound encounter complex (equilibrium 25.23). 

Positive AK suggests dissociative D or Zj) the rate law suggests associative mechanism apply Eigen-Wilkins mechanism to account for apparent second order kinetics. 

Picolinic acid (pyridine 2-carboxylic acid) complexes of chromium(III) have been the subject of a number of studies. Complexation by picolinic acid in water/ethanol (30% v/v) follows an ion-pairing, Eigen-Wilkins type mechanism.Activation parameters suggest an associative character for the reaction of the aqua complex. Chelated complexes of chromiuni(ni) and picolinic acid are the products of the rapid, inner-sphere reduction of [Co (pico)(NH3)5p with chromium(II). The reaction of the related 4-carboxylic acid complex of cobalt(III) with chromium(II) is also rapid in contrast, pyridine-3-carboxylic acid (nicotinic acid) complexes undergo slower reactions. A -hydroxy-bridged dimeric complex [Cr2(pico)4(OH)2] has also been prepared. A study of magnetic properties in the temperature range 16-300 K leads to J - —6 cm and g = 2, typical for such complexes. 

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