The Irving-Williams Series

Some typical stability constant data are presented in Fig. 8-14, whilst Fig. 8-15 shows some biological manifestations that illustrate the ubiquity of the effect. The sequence is known as the Irving-Williams series. 

Can we rationalize these observations in terms of ligand-field or other effects The data that we have presented in Fig. 8-14 refers to the log Ki values for each ligand with the high spin divalent metal ions. The sequence reflects a number of simple properties of the cations. Firstly, the trend closely parallels the ionic radii 

However, consideration in terms of the ionic radius or the LFSE shows that both factors predict that the maximum stabilities will be associated with nickel(ii) complexes, as opposed to the observed maxima at copper(ii). Can we give a satisfactory explanation for this The data presented above involve Ki values and if we consider the case of 1,2-diaminoethane, these refer to the process in Eq. (8.13). 

The first feature that we note is the relative destabilization of the [CuLsl complexes compared to the marked stabilization depicted in Fig. 8-16. 

The data for the 1,2-diaminoethane complexes now parallels the trends in ionic radius and LFSE rather closely, except for the iron case, to which we return shortly. What is happening Copper(ii) ions possess a configuration, and you will recall that we expect such a configuration to exhibit a Jahn-Teller distortion - the six metal-ligand bonds in octahedral copper(ii) complexes are not all of equal strength. The typical pattern of Jahn-Teller distortions observed in copper(ii) complexes involves the formation of four short and two long metal-ligand bonds. 

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As anticipated, the complexation is characterised by negative p-values, indicating that the binding process is favoured by electron donating substituents. The order of the p-values for complexation of the different Lewis-acids again follows the Irving-Williams series. 

Figure 8-15. The biological activity of some transition-metal ions illustrating the Irving-Williams series.

Our discussion of the Irving-Williams series illustrates, as ever, an important generalization in transition-metal chemistry in many cases there is no single, simple principle which may be invoked to rationalize a given series of observations. Whilst LFSE effects are very important, they are but one of several factors controlling structure and thermodynamics. 

Some of these divalent cations form part of the Irving-Williams series Mn, Fe, Co, Ni, Cu and Zn. Irving Williams (1953) examined the stability constants of complexes of a number of divalent ions and found that the order... 

Breslow demonstrated the catalytic effect of having not only a Lewis acid zinc center but also an auxiliary catalytic center held in close, but non-binding, proximity. An imidazole or thiophenol (81) demonstrated increase in effectiveness in cyclization of a phosphate derivative.706,707 Pyridine pendents on this macrocycle gave a stability for zinc in line with the Irving Williams series for two pyridine and three pyridine pendents.708 A contrast was noted with other metals (Ni, Cu, Cd, Pb, Fe, and In), all of which increase in stability with three pendent arms as does zinc this was attributed to coordination preferences. 

Fig. 2.8. The variation of stability constants, K, for the complexes of M2+ ions of the Irving-Williams series, ox, oxalate en, ethylenediamine (note that the constants plotted are absolute, not effective constants at pH = 7).

We now look at the values of the free M concentration and hence to the binding strength to selected A synthesised in the cell. The constants are closely common to all cells in their common compartment, their cytoplasm. The values, suited to metabolism, can be put in series in which Na+ and K+ bind poorly and only to a few of the weakest donors based on neutral O-donor centres while other metal ions bind more strongly to O, N and S donors of proteins or small organic molecules in a well-recognised order, i.e. in the Irving/Williams series (see Section 2.17) ... 

When a graph is made of log fCj versus the sum of the first and second ionization potentials for the metals, the result is shown in Figure 19.5. Clearly, the relationship is linear for the metal ions except Zn2+, for which the complex is considerably less stable than that with Cu2+. These results are in accord with the Irving-Williams series described earlier. The graph obtained when log K2 is plotted against the total ionization potential has exactly the same characteristics, as is shown in Figure 19.6. 

The second study is relevant to a discussion in the paper by Dr. Wilkins of instances in which the relative rates are related to stabilities. We have come across a very striking example of such a relationship in the activation of the enzyme ribo-nuclease by metal ions. In Figure E the activity is plotted vs. concentration of metal ion added to the enzymatic reaction mixture. In the absence of metal the activity is as indicated by the straight line in the center of the figure. It can be seen that a list of the transition metals in the order of concentrations giving maximum activity corresponds rather neatly with the Irving-Williams series. 

Not mentioned in Table 2 (and often not in the original papers ) is the optical form (chirality) of the amino acids used. All the amino acids, except for glycine (R = H), contain an asymmetric carbon atom (the C atom). In the majority of cases the optical form used, whether l, d or racemic dl, makes little difference to the stability constants, but there are some notable exceptions (vide infra). Examination of the data in Table 2 reveals (i) that the order of stability constants for the divalent transition metal ions follows the Irving-Williams series (ii) that for the divalent transition metal ions, with excess amino acid present at neutral pH, the predominant spedes is the neutral chelated M(aa)2 complex (iii) that the species formed reflect the stereochemical preferences of the metal ions, e.g. for Cu 1 a 2 1 complex readily forms but not a 3 1 ligand metal complex (see Volume 5, Chapter 53). Confirmation of the species proposed from analysis of potentiometric data and information on the mode of bonding in solution has involved the use of an impressive array of spectroscopic techniques, e.g. UV/visible, IR, ESR, NMR, CD and MCD (magnetic circular dichroism). 

For the divalent metals, the order is largely as expected from the Irving—Williams series of stability constants. A similar extraction order has been reported for solutions of salicylaldoxime (14 R = R = H) in benzene, although the extraction of zinc(II) was found to be anomalously... 

The stabilities of these metal complexes follow the Irving-Williams series (38), but Co2+ and Ni2+ are less strongly bound than Zn2+ (Table 7). The logarithm of the apparent stability constant for zinc increases linearly and with unit slope between pH 5.5 and 10. No significant differences were found for the human and bovine forms, respectively. 

Zinc and other metal ions have been found to promote pyrrole hydrogen ionization in 2-(2 -pyridyl)imidazole.269 Complexation studies114 on the systems N-methyl-histamine and NN-dimethylhistamine with bivalent zinc, copper, cobalt, and nickel have shown that the stabilities of the complexes follow the Irving-Williams series. With respect to the variation of a ligand with the same metal ion, the stability decreases in the series histamine, N-methylhistamine, NN-dimethylhistamine, possibly as a result of steric hindrance. The complexes are assigned the structure (12). 

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