Trends in stepwise stability constants

For a given aqueous solution containing known concentrations of a metal ion and hgand L, it may have been found that only one coordination complex of known formula is present in solution. If this is the case, then the stability constant for this complex can be obtained directly from a determination of the concentration of uncomplexed L or complexed in that solution. Such determinations can be made by polarographic or potentiometric measurements (if a suitable reversible electrode exists), by pH measurements (if the hgand is the conjugate base of a weak acid), or by ion-exchange, spectrophotometric (i.e. observation of electronic spectra and use of the Beer-Lambert Law), NMR spectroscopic or distribution methods. 

In the past, the use of empirical methods to predict stability constants for metal ion complexes has had only hmited application. The use of DFT theory (see Box 5.2) to calculate values of AG for the gas-phase equilibrium  

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There are, however, a few examples in which the initial complexes are less stable than their more highly substituted relatives. Deviations from a regular decreasing trend of stepwise stability constants have been interpreted in some cases as indicating a change in the coordination number of the metal ion. The stability constants for [CdBr4] sxs K = 2 r- 10, Ki = 6, = 0.6, = 1.2. 

Figure 6.11 shows that for the formation of the complex ions [Al(H20)g xFx] (x = 1-6), the stepwise stability constants become smaller as more F ligands are introduced a similar trend is also observed in the formation of [Ni(H20)g x(NH3)x] (x = 1-6) in worked example 6.6. This decrease in values of K is typical of many systems however, the trend is not always as smooth as in Figure... 

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