Complex formation Eigen mechanism

The interpretation of the ultrasound relaxation data is based on the assumption that the rate-controlling step is the loss of water from the cation solvation shell via the Eigen mechanism of complex formation (Eigen and Tamm 1962)... 

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,... 

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. 

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... 

The Homogeneous Case. Margerum (1978) and Hering and Morel (1990) have elaborated on mechanisms and rates of metal complexation reactions in solution. In the Eigen mechanism, formation of an outer-sphere complex between a metal and a ligand is followed by a rate limiting loss of water from the inner coordination sphere of the metal, Thus, for a bivalent hexaaqua metal ion... 

The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-1 s-1.642 Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x 103 and 1.13 x 103 mol-1 s 1 for MDA- and VMDA ) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii Table 27), species with coordinated hydroxyl are labilized. 

Relaxation of complicated ligands may occur as a step in both pathways. Diebler and Eigen 461 indicated the ways in which such mechanisms could be analysed using fast reaction methods. Several studies of Ln(III) complex formation and of the formation of Ln(III) mixed complexes have been analysed. Generally the dissociative mechanism is considered to dominate and we are then concerned with the water exchange rate. Several studies have shown that the rate decreases from La(III) to Lu(III) but there seems to be a minimum rate around Tm(III). This is also seen in the rate of rotation of ligands on the surface of the ions, Fig. 7. There may be a small crystal field term, or another contribution to a tetrad -like effect from the 4f electron core. However in the hydrate the precise relationship between the inner and outer sphere water may also be important as we saw when we discussed the heat and entropy of complex ion formation. 

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)... 

The data were analysed in terms of a three-step complex formation mechanism of Eigen and Tamm [18]. The mechanism consists of successive removal of one water molecule from the solvation sheath of each ion in three reversible steps as the ions approach from infinite separation to ultimate contact. 

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... 

For labile complexes, it is often quite difficult to distinguish between inner and outer sphere complexes. To add to this confusion is the fact that formation constants for such labile complexes when determined by optical spectrometry are often lower than those of the same system determined by other means such as potentiometry, solvent extraction, etc. This has led some authors to identify the former as "inner sphere" constants and the latter as "total" constants. However, others have shown that this cannot be correct even if the optical spectrum of the solvated cation and the outer sphere complex is the same (4, 7). Nevertheless, the characterization and knowledge of the formation constants of outer sphere complexes are important as such complexes play a significant role in the Eigen mechanism of the formation of labile complexes (8) This model describes the formation of complexes as following a sequence ... 

Eigen, M., and R. G. Wilkins, in Mechanisms of Inorganic Reactions, Adv. in Chem. Series, No. 49, p. 55, American Chemical Society (a recent survey of complex formation studies, with extensive references to literature on these and other fast reactions of complexes). 

More elaborately, complex formation can be viewed as in Fig. 2 (the detailed kinetic implications of this refinement to the basic Eigen mechanism are covered by Benton and Moore [71]). It is reasonable to relate the rate coefficient for the dissociative slow stage with that for solvent exchange, since breaking of a metal ion—solvent bond is involved in both processes. 

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

Prior to the advent of variable pressure 0 n.m.r. studies of solvent exchange in the aqua ions of V(II) to Ni(II) it had generally been accepted that the complex formation reactions on all octahedral divalent ions proceeded via Ij mechanisms. However, it now appears that for the early members of the series the associative I, mechanism becomes important with the reactions showing a small dependence on the entering ligand L. The usual Eigen Wilkins approach still appears to be effective as a predictive tool for rationalising the rates of both dissociative and associative I, substitution reactions in these cases. 

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. ... 

The complexation reactions usually proceed by the Eigen mechanism (Diebler and Eigen 1959, Eigen 1963, Eigen and Tamm 1962). This mechanism involves two steps, the rapid formation of an outer-sphere association complex (i.e., an ion pair) and the subsequent rate-determining step in which the ligand displaces one or more water molecules. 

The values of log/Cj and 0gk2 are 8.85 and 7.70, respectively. Similar conclusions were reached for the tartrate (Yun and Bear 1976), anthranilate (Silber et al. 1969) and picolinate (Erikson et al. 1987) complexation and for mixed-complex formation involving Ln (EDTA)(X) where X is a picolinate (Ekstrom et al. 1980) or a 5-sulfosalicylate anion (Ekstrom et al. 1981). The kinetic parameters for the overall reactions of these systems (Erikson et al. 1987) were consistent with the Diebler - Eigen mechanism (diebler and Eigen 1959, Purdie and Farrow 1973),... 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

The work being reported in this paper is part of a continuing study on metal-complex formation using BeSO as a test electro lyte. In pure aqueous solution the reaction has been extensively studied (1) and is shown to follow the Eigen-Tamm mechanism ... 

In the particular case of substitution reactions of [Fe(CN)5(OH2)], ligand replacement is formally a complex formation reaction, and indeed the standard Eigen-Wilkins kinetic pattern and mechanism applies, albeit to what appears to be a negatively charged aquo ion Values of observed second-order rate constants, with and values where available, are listed in Table As... 

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