Complex Formation with Chelating Ligands

In this section we shall consider the data which have been presented recently on metal complexation reactions involving bi- and multi-dentate ligands. It has frequently been assumed (and, indeed, there are many 

In some cases, however, there is a marked difference between the kf values for mono- and bi-dentate ligands reacting with the same metal ion, 

Lin and Bear report a pressure-jump investigation of the formation of magnesium oxalate in water. Rate constants and activation parameters are quoted and discussed in terms of the normal mechanism. The vanadyl-tartaric acid and nickel(ii)-8-hydroxyquinoline and -8-hydroxyquinoline-5-sulphonate systems are also reported. An n.m.r. study involving the reaction of with Mn +, Fe +, Co +, and Ni + suggests that the mechanism involves several parallel pathways.  

Diebler has studied the reactions between bipy and Cr + or Cu + and found them to be sensitive to pH. From the pH-dependence of the relaxation times, he was able to show that the mechanism involves the reaction of the metal ion with either the protonated (Hbipy+) or unprotonated (bipy) form of the ligand. He obtained the overall rate constants given in Table 3 

Eigen s group have given a general account of the kinetics and mechanism of reactions of main-group metal ions with biological carriers, in which they include the results of a study into the binding of alkali-metal ions with murexide (2) see Chapter 2, Section 1. 

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Fig. 8.5. Examples of complex formation by chelating ligands, showing (a) a four-membered ring with diethyldithiocarbamate (dtc), (b) formation of five-membered rings with ethylenediaminetetraacetate EDTA and (c) a six-membered ring with acetylacetonate (acac).

Gorski B, Koch H (1970) Technetium complex formation with chelate-forming ligands. II. J Inorg Nucl Chem 32 3831-3836... 

Table 40 lists some commonly used ligands in chelate extraction. All these ligands are polyprotic acids which mean that the metal complex formation with these ligands depends on the pH of the solution. 

A triply chelating ligand is the purpurate anion, (/), the ammonium salt of which is the indicator murexide. The kinetics of its complex formation with alkali metals have been reviewed (75). In the lithium salt the cation is 5-coordinated in a square pyramid with the base formed by the three donor atoms, 0, N, O from one anion and a carbonyl oxygen from... 

In the case of Fem complexes with chelating ligands, generation of ligand radicals is observed. Thus, in the case of the Fem complexes with aminopolycarboxylic acids, eg [Femedta] , photoreduction of Fem - Fe11 is accompanied by decarboxylation and formation of the respective organic radical [11,20-22. ... 

Table 2.4 Last complex formation constants and corresponding modified c and x values for octahedral complexes of Fe(lll) and Co(ll) with chelating ligands...

The reactivity of zinc complexes supported by a variety of tridentate amine donor ligands (Fig. 44) with diphenyl 4-nitrophenyl phosphate in 20% (v/v) acetonitrile-water, and with 2,4-dinitrophenyl diethyl phosphate in 1 % (v/v) in methanol-water, has been investigated. 5 For the former reaction, the second-order rate constant for the hydrolysis of 2,4-dinitrophenyl diethyl phosphate correlates linearly with the —AH value for the formation of the [(ligand)Zn(OH2)]2 + complex from free chelate ligand and aqueous zinc ion. This indicates that in this series of complexes, faster hydrolysis of 2,4-dinitrophenyl diethyl phosphate corresponds to weaker chelate... 

Tc pharmaceuticals based on coordination complexes with functionalized ligands are also known as Tc essentials those concerning labeled particles and macromolecules are called Tc-tagged radiopharmaceuticals. A variety of chelating agents have been developed for complex formation with certain oxidation states of technetium, providing the structural requirements for uptake and retention (Schwochau 2000). Examples of Tc essentials are shown in Fig. 2.3.1. 

The appearance of polynuclear complexes sometimes facilitates polymerization and formation of large macromolecules, which are capable of making solution into colloid. For instance, at hydrolysis of oxide iron Fe may form a complex compound Fe(OH)/, which polymerizes and forms large colloid molecules [Fe(OH)j]. In such solutions precipitates mineral of iron hydroxide - the limonite. Similar colloid forms occurrence are typical of many chelate complex compounds with organic ligands. 

Zinc associated with purified enolase molecules appears to involve complex formations with or without chelation. Most of the zinc associated with the enolase molecules can be removed by electrodialysis and restored to the molecule, even after denaturation. In contrast, one of the zinc atoms is tightly bound and cannot be removed by electrodialysis, nor can it be replaced in the enolase molecule after its denaturation with 4 M urea. The last of these observations illustrates one important property of the protein ligand in biological chelation. 

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