Chelates, dissociation

In the equation for pM, log K appears instead of piC because iCis a formation constant, the reciprocal of the chelate dissociation constant, which is analogous to the acid dissociation constant K. ... 

The alternative mechanism to CO dissociation, proposed by Stufkens (23) for the DAB complexes, is not consistent with the difference between thermal and photochemical reaction products, Equation 11. In solution Kokkes et al. propose that one end of the DAB chelate dissociates on photolysis. If this were the case it would be difficult to understand why the photochemical reaction (where the DAB ligand is half attached) leads only to CO displacement, while the associative thermal reaction leads only to DAB displacement. Consider the mechanism, Equation 13, established (19) for thermal loss of DAB. The key to DAB loss is formation of the mono-dentate species D of Equation 13. This intermediate is identical to that proposed by Kokkes et al. (23) for photochemical CO replacement. According to their mechanism, Equation 14, the same species D, forms in a two step process and would therefore be thermally equilibrated. Thus the alternative mechanism is not consistent with thermal chemistry of these systems. 

An example of a reaction in which the reduced form of the metal ion is converted back to its higher oxidation state by molecular oxygen, is the autoxidation of ascorbic acid by copper(II) (206, 207). The probable course of the reaction is as follows The ascorbate ion forms an intermediate copper(II) chelate which undergoes an internal oxidation-reduction reaction, thereby forming a copper(I) semiquinone chelate. Dissociation of the relatively unstable copper(I) chelate occurs and the copper(I) ion is oxidized by molecular oxygen and the semiquinone is oxidized by molecular oxygen or copper(II) (160). 

Mechanisms and Rates of Lead-Chelate Dissociation and Substitution Reactions... 

In addition to a detailed discussion of hydration dynamics of the lanthanides, Lincoln s review also covers the kinetics of solvation in nonaqueous media and complexation kinetics. As our focus is on the aqueous chemistry of the lanthanides, we will not discuss recent developments in nonaqueous lanthanide solution chemistry. The intervening four years have seen the publication of a handful of studies of the kinetics of lanthanide chelate dissociation kinetics. In the following section we will discuss the best of these results. 

Most of the quoted rate constants are at I = 0.1 and 25°C, kf = rate constant for chelate formation, kj = rate constant for chelate dissociation. 

If the complex is an octahedral bis chelate system, M(AA)2(X>2, then two trigonal bipyramidal intermediates are possible, as shown in Scheme 4.3. The product distributions in Scheme 4.3 ate based on the assumption of a statistical attack along the edges of the trigonal plane of the intermediate. Since the lower intermediate has a plane of symmetry, it must lead to racemization. Similar intermediates can be drawn for tris chelate systems if one end of a chelate dissociates. This is called one ended dissociation and is actually an intramolecular process. 

When considering the kinetic stability of a lanthanide chelate, dissociation and trans-metallation reactions are the main processes that must be taken into account. For complexes formed with EDTA, the proton-assisted dissociation dominates over direct attack by another metal ion. Indeed, the structure does not leave space for direct attack, because none of the acetate arms remains exposed [7]. This can be contrasted with the DTPA case where the two residual negative charges on Ln(DTPA) make the direct attack reaction more likely. A study by Briicher et al. [8] demonstrated that at physiological pH the transmetallation mechanism dominates and that the kinetics of the process largely depends on the attacking... 

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