Kinetics of chelate formation

Complexes containing bidentate, tridentate and so on ligands are very common with metal ions. They are especially important because of their apparently enhanced stability, often referred to as the chelate effect . Such ligands have two or more potential donor atoms separated by two, three or four other atoms in the molecule. Some common examples are oxalate, glycine, salicylate, ethylenediamine, acetylacetonate, 2,2 -bipytidyl and ethylenediaminetetraacetic acid. 

If the monodentate species (M—L—LH and M—L— L) are assumed to be reactive intermediates, so that the steady-state approximation can be applied to their concentrations, and the usual pseudo-first-order conditions of [M] [L] and constant [H ] are assum, then it can be shown that the pseudo-fiist-order rate constant is given by Eq. (3.76) for [H J. 

Rearrangement of this equation gives Eq. (3.77), which is of the form often used to analyze data by plotting the left-hand side versus [ ] .  

In a typical study, such as that of Wilkins and co-workers, this plot is linear. The traditional explanation for this is that 35 2,[HTand 35 34, SO that Eq. (3.77) simplifies to Eq. (3.78), which will give k 2 as the intercept and slope. 

These and other problems caused Letter and Jordan to reconsider the assumptions used to reduce the complete rate law. Is it reasonable that ki t If one uses the Ni(II)-glycine system for analysis, then an estimate of i( 2i HI can be obtained from the corresponding nickel(II)-acetate system, and ((HjhOsCo—OjCCHjNHj ) can be used to estimate that K[ 3xlO °M. Tlien, the condition that k k2t[H l pH 6 requires that Jfejj 10 s . However, k = 3x10 s for water exchange on Ni(II), and all the available evidence indicates an mechanism for substitution on Ni(II). Therefore, if is 10 s th die water ligands in the monodentate intermediate must be labilized by about a factor of 10, an effect that is without precedent 

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