Irving-Williams series stability

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. 

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).

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). 

The marked lability of manganese(II) species is undoubtedly one of the reasons for the neglect of manganese chemistry. This lability, often coupled with low stability of species (see Irving-Williams Series)29 ensures that there is little correlation between solutions and solids, and adds an uncertainty to this area of chemistry that is largely absent from the chemistry of inert compounds. 

A second difficulty in studying Mn proteins is the lability of protein-bound Mn. Complexes of Mrf typically have binding constants that are somewhat smaller than those for the corresponding Fe complexes and substantially smaller than those for Cu and Zn complexes (this is the so-called Irving-Williams Series). For heme-iron proteins, the porphyrin cofactor see Iron Porphyrin Chemistry) increases the stability of the Fe-protein interaction. No such small molecule cofactors are known for Mn proteins. Small binding constants mean that Mn is readily lost from a protein during purification. 

Comparison of the stability constants for Zn + complexes with those for the preceding transition metal M + cations shows conformance to the Irving-WUhams stabihty order (see Irving-Williams Series) (Mn + < Fe + < Co + < Ni < Cu + Zn +) for essentially all hgands. As an example. 

Figure 6.11. Stability constants of 1 1 complexes of transition metals and solubility pr ucts of their sulfides (Irving-Williams series).

For some time coordination chemists were aware of certain trends in the stability of metal complexes. One of the earliest correlations was the Irving-Williams series of stabilityFor a given ligand, the stability of complexes with dipositive metal ions follows the order Ba " " < Sr < Ca"" < Mg < Mn " < Fe < Co" < Ni < Cu > Zn . This order arises in part from a decrease in size across the series and in... 

Formation constants for many metal complexes have been compiled by Ramunas Motekaitis and Art Martell, and these as well as techniques for measuring them in the laboratory will be covered in Chapters 3 and 8. One can, however, predict the relative stability of a desired complex based on simple bonding theories. Crystal field theory, as well as the Irving-Williams series and Pearson s hard-soft-acid-base theory (see the next section) enable us to predict what might happen in solution. 

The Martindale s Extra Pharmacopoeia lists more than 40 preparations for iron, but only three for zinc some of the most frequently used iron and zinc supplements are illustrated in Table 5.1. Irritation of the gastrointestinal tract tends to follow the Irving-Williams series of complex stability, i.e. 

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