The Eigen-Wilkins mechanism

Water exchange is always more rapid than substitutions with other entering ligands. Let us now consider reaction 25.22. 

The mechanism may be associative (A or f) or dissociative (D or f), and it is not at all easy to distinguish between these, even though the rate laws are different. An associative mechanism involves a 7-coordinate intermediate or transition state and, sterically, an associative pathway seems less likely than a dissociative one. Nevertheless, activation volumes do sometimes indicate an associative mechanism (see Table 25.4). However, for most ligand substitutions in octahedral complexes, experimental evidence supports dissociative pathways. Two limiting cases are often observed for general reaction 25.22  

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). 

Usually, the rate of formation of MLg,Y and the back-reactiOTi to MLg and Y are much faster than the subsequent conversion of MLg,Y to products. Thus, the formation of MLg,Y is a pre-equilibrium. The equilibrium constant, K, can rarely be determined experimentally, but it can be estimated using theoretical models. The rate-determining step in the Eigen-Wilkins mechanism is step 26.27 with a rate constant k. The overall rate law is eq. 26.28. 

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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). 

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... 

Metal Complex Formation - The Eigen-Wilkins Mechanism [10, 19,29]... 

A r-jump and stopped-flow study of the rapid anation of a five-co-ordinate copper(ii) aqua-complex by azide and thiocyanate has its main interest as a model system for the active site of a metalloenzyme. The kinetics conform to the Eigen-Wilkins mechanism (rate-determining water release, /d). ... 

NickeI(II) (rf ).— The mechanism of complex formation for some metal(ii) cations is now so well established, at least for simple ligands, that such reactions, particularly of nickel(n), are used to probe solvent and salt effects on kinetic patterns. Many of these studies are therefore dealt with in the Chapter on medium effects (Part II, Chapter 13). They include the reactions of nickel(n) and of magnes-ium(n) with chloride in aqueous alcohols, of nickel(ii) with imidazole in aqueous ethanol, with malonate in fructose-water solutions, with thiocyanate in methanol-DMSO mixtures, and with murexide or pada in various micellar media, and of several metal(n) cations with fluoride in aqueous salt solutions." In general, medium effects on observed rate constants (lit) for complex formation operate on the pre-association step (/fos) rather than on the interchange process (A i). The Eigen-Wilkins mechanism operates in all these media it has also been shown to operate for the bidentate ligand Etgdtc in DMSO, and even at the surface of a mercury electrode. ... 

Reactions between Ni (aq) and bidentate ligands in aqueous solution generally take place according to the Eigen-Wilkins mechanism, (See the paper by Greenwood, Robinson and White (5) for a fuller discussion.) Formation of the monodentate complex is rate-limiting. Since the observed rates are... 

The study of the effect of solvent upon the rate of complexation has revealed details of the reaction mechanism, and it is hoped that a study of the solvent-dependence of activation volumes may permit further elucidation of metal-ligand, metal-solvent and solvent-solvent interactions. According to the Eigen-Wilkins mechanism for ligand substitution reactions ( )(5)5 the first step is formation of an outer-sphere complex, characterised by an equilibrium constant Ki2 Subsequently the ligand enters the first coordination sphere and the forward rate constant for this step is identified with kg for exchange of... 

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