The Formation of Chelates

The successive steps in the replacement of two coordinated waters by a bidentate ligand L-L is represented as 

Assuming stationary-state conditions for the intermediate, in which L —L is acting as a unidentate ligand, we find 

The function 2/ -1 will dominate the kinetics of bidentate chelation. 

The establishment of the first bond appears to signal rapid successive ring closures with most of the multidentate ligands examined. (However, consider the Ni(II)-fad system. Sec. 1.8.2) In certain cases the later steps in chelation can be shown to be more rapid than the earlier ones, by clever experiments involving laser photolysis (Table 3.4) or pH-adjustments of solutions containing partially formed chelates.  

The dissociation rate constant is now composite, = k ik 2/k2- Following the first bond rupture (k 2) the competition between further bond rupture (/ , ) and reformation (Atj) which may lead to a small k j/k2 is the basic reason for the high kinetic stability of the chelate. The problem of complete dissociation is intensified when complexes of ligands of higher dentate character are examined. The situation is altered when the successively released donor atom(s) can be prevented from reattachment (see subject of accelerated substitution). 

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The situation is more complex, when there is an additional oxygen functionality in the a -po-sition (see Table 18)100,101. In this case the a -oxygen may also be involved in the formation of chelates, such as 28 and 29. Thus, chelation with the a -oxygen atom is expected to favor the diastereomer opposite to that from formation of an a-chelate. 

Figure 7.30 Mechanism of phenol and formaldehyde reaction using base catalyst involving the formation of chelate.

NMR investigations of the BC13 systems with TMP and TBP indicate the formation of chelates and of alkyl chloride. 

The linking of a metal to an antibody could, in principle, be accomplished by forming the metal chelate either prior to or after attachment to protein. Success to date has been achieved only by formation of the protein-ligand conjugate before metal chelation. The complexation reaction has several general features. First, reactions between the metallic radionuclides and antibodies are almost always performed with sub-stoichiometric quantities of chelate and metal ion. It is therefore of the utmost importance that no carrier added metals obtained from commercial sources be exceedingly pure or else be purified prior to use. Reactions of "carrier added" metal solutions are not likely to be of use because of the ease with which available chelate sites become saturated. Because the formation of chelate complexes is usually a bimolecular reaction, the complexation will proceed optimally when more chelation sites are available. Similarly, the more isotope in solution, the faster the reaction. Employment of a carrier chelate to insure solubilization of the radiometal is of value to maximize available isotope and the acetate ion has proven useful. 

Abstract—Ultraviolet and infrared spectroscopic evidence is presented on the formation of chelate rings involving hydrogen bonds between the groups mentioned in the title and substituted in paraffinic radicals. 

Involvement of two nucleophilic nitrogen atoms is thus typical for the amino heterocycles. The mutual disposition of the pyridine and amine nitrogen atoms allows the formation of chelate structures for the cobalt complexes of purine, 221 and 222. Structures with the N, iV -five-membered metal cycles were proven for the tri- and tetranuclear complexes of silver ) with 8-aminoquinoline (223) (92IC4370), and polymeric copper- and rhodium-acetate clusters (224). Another coordination mode can be found in the complexes of 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole, (225 or... 

Schiff bases having two nitrogen atoms as donors may be derived either from condensation of dialdehydes and diketones with two molecules of an amine, or from reaction of diamines with aldehydes or ketones. In Section 20.1.2.1, it has been pointed out that coordination through the N atom may occur only under particular circumstances. However, in the case of diimines the formation of chelate rings stabilizes the metal-nitrogen bond. Thus, they can form both mono-41 and bis-chelate42 complexes. 

Rate enhancements of 104 - 106 are typically associated with the formation of chelated complexes in which the carbonyl oxygen atom is also co-ordinated to the metal (3.2). This results in a considerably greater polarisation of the C-0 bond. 

A full report for the identification of salicylamide through melting point, reactions of the amide group, the phenolic group, the benzene ring, and the formation of chelate compounds has been published (9). 

The formation of chelate complexes with one bidentate ligand replacing two monodentate ligands is enhanced by the entropy increase when the number of molecules increases. 

The formation of chelated species such as D (Fig. 13) would better explain the stereochemical results. 

The dicarboxylates have been separated from the compounds of Section 41.3.5.2(ii) because of the possibility of the formation of chelate rings (106). Such chelation, however, is by no means universal in these compounds. The 1 1, 1 2 and 1 3 species, which have been shown to exist in aqueous... 

Simple inorganic ions and molecules like NH3, CN , Cl-, H20 form monodentate ligands, that is one ion or molecule occupies one of the spaces available around the central ion in the coordination sphere, but bidentate (like the dipyridyl ion), tridentate, and also tetradentate ligands are known. Complexes made of polydentate ligands are often called chelates, the name originating from the Greek word for the claw of the crab, which bites into an object like the polydentate ligand catches the central ion. The formation of chelate complexes is used extensively in quantitative chemical analysis (complexometric titrations). ... 

Oxyacids, like citric or tartaric acids, and polyols, like saccharose are also used, mainly as masking agents, in qualitative analysis. The action of some specific reagents, like oc-a -bipyridyl for iron(II) and dimethylglyoxime for nickel(II), is also based on the formation of chelate complexes. In quantitative analysis the formation of chelates is frequently utilized (complexometric titrations). ... 

Antacids are gastric-acid neutralizing or adsorbing medications, usually containing aluminium and/or magnesium salts (e.g. aluminium hydroxide, magnesium carbonate, hydroxide or trisilicate), which have the ability to decrease the absorption of several medications that are co-administered (often due to the formation of chelates/insoluble unabsorbable complexes). 

Schwarz et al. [326] synthesised a functionalised bis-imidazolium salt with hydroxy end groups on the wingtips [327,328] and used it in the formation of chelating cw-bis-carbene complexes of palladium(ll) applied as catalysts in the Heck reaction. The functional groups were needed to immobilise the catalyst by attachment to a polymeric support [329] (see Figure 3.104). 

The presence of additional Lewis base sites within the molecule can result in the formation of chelates with SnCU or TiCU, which can lead to 1,2- or 1,3-asymmetric induction with the appropriate substitution at the C-2 or C-3 center. NMR studies have provided a basis for explaining the levels of diastereofacial selectivity observed... 

Platinum(II) complexes have antitumor activity, and have been tested against P-388 leukemia (derivatives of the substituted o-phenilenediamine). Their antitumor activity has been connected with many factors. These include the formation of chelate rings and their strength, the nature and the influence of different substituting groups, and the relative stability of the Pt(II) complexes. ... 

These complexones, as they are often called, form very stable water-soluble complexes with even the Gp. IIA metals (p. 263). With Gp. Ill metals the stability of the complexes is even more marked, indeed a pH of 13 can be reached without precipitating the rare earth oxides when (a) or (b) is present (p. 431). The stabilities are associated with the formation of chelate rings. 

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