Statistical and chelate effects

The variations in stability constants which have been discussed so far in this chapter are typically rather large. Superimposed on them are some smaller but systematic variations which also merit attention. 

Most stability constant measurements are made in aqueous solutions at a constant ionic strength. The species we have called M must, in reality, be a complex mixture of species. The majority will probably be [M(H20) ] , where n may well be four or six although larger numbers also occur. Other 

In this reaction it has been assumed that the number of ligands around M is constant, and equal to (m + n). If we let this number (usually four or six) be N, then the reaction may be written as 

As has been seen in Table 5.1, it is commonly found that for a given M and L there is a decrease in successive formation constants, K2. .. or p2. .. This is largely a statistical effect. It is easier to attach L to MLJ than to MLJv-iL because there are JV reaction sites on the former but only N -1 on the latter (replacement of L in by L gives an identical 

The rate of formation of MLJv iL +i is, for a simple one-step reaction, kf [ML L ][L]. If the statistical effect is the only one which varies as n changes, the forward rate constant will vary with n in proportion to the number of reaction sites, that is, the number of V ligands in We 

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Kiessling et al,48 reported a similar increase in the inhibitory activity of synthetic polymers of increasing lengths that possess Man residues in a hemagglutination assay with ConA. The authors concluded that the enhanced inhibitory activities of the longer chain polymers were largely due to a combination of statistical and chelation effects and therefore slower dissociation rates. 

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