Metal complexes chelation mechanisms

A similar reaction mechanism was proposed by Chin et al. [32] for the hydrolysis of the biological phosphate monoester adenosine monophosphate (AMP) by the complex [(trpn) Co (OH2)]2+ [trpn = tris(ami-nopropyl)amine]. Rapid cleavage is observed only in the presence of 2 equiv metal complex. It is evident from 31P NMR spectra that on coordination of 1 equiv (trpn)Co to AMP a stable four-membered chelate complex 4 is formed. The second (trpn)Co molecule may bind to another oxygen atom of the substrate (formation of 5) and provide a Co-OH nucleophile which replaces the alkoxy group. The half-life of AMP in 5 is about 1 h at pD 5 and 25 °C. 

In a bidentate ligand system, three molecules of a dye containing either a terminal salicylic acid unit (as in 5.2) or an o-nitrosonaphthol residue are able to chelate simultaneously with a trivalent metal ion of CN6, such as chromium (III) or iron(III), to form a 1 3 metal-dye complex (as in 5.8). Historically, the most important bidentate ligand system was alizarin (5.1). It has been suggested that both hydroxy groups and the keto group in the peri position are all involved with the metal atom in the chelation mechanism. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

A complete systematic description of protein-metal complexation has yet to be presented, but it is apparent that many mechanisms are involved. Some proteins may participate in classical chelation interactions via polycarboxy clusters on their surfaces.2 Others interact with metals via coordination with polyhis-tidyl or other aromatic domains.13 5 Still others may interact with metals via sulfhydryl residues.13 The literature on immobilized metal affinity reveals examples of unexplained retention that may involve yet other mechanisms.1... 

The decarboxylation of simple /f-ketoacids, such as acetoacetic acid, is not metal promoted (Fig. 5-22) - this is in part due to formation of the chelate complex, which is in the enolate form. Mechanistic studies have indicated that the enol or enolate is inactive in the decarboxylation reaction. The mechanism indicated in Fig. 5-21 is not applicable to the metal complex. 

Several reviews have recently appeared which cover the topics of inter-1 6 and intramolecular1, s-n rearrangement reactions of metal complexes. In order to minimize duplication this review will be limited to intramolecular metal-centered rearrangementjeactions of six-coordinate tris-chelate complexes. Both kinetically slow (rates < 10 2 sec"1) and fast (rates % 10" 2 sec"1 )b complexes will be considered mainly from a mechanistic point of view. In addition various structural and electronic parameters of tris-chelate complexes will be scrutinized in order to assess their affect on the rate and mechanism of rearrangement. 

A topic related to the foregoing discussion concerns metal complexes, such as many sulfur-containing metal chelates,357 364 that are capable only of inhibiting autoxidations. The detailed mechanisms of the inhibiting action of these metal complexes are not very well understood. Recent results362 suggest that zinc dialkyldithiophosphates react with alkylperoxy radicals at the metal center, which could involve electron transfer or an SH2 reaction ... 

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