Metal ions ligand exchange processes

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

It is well known that the chiral resolution of these CSPs occurred as a result of the exchange of ligands and enantiomers on the same metal ion. Therefore, these CSPs are suitable only for those racemates which can coordinate with the metal ion. Therefore, racemates like amino acids, amines, and hydroxy acids have been resolved successliilly by the ligand-exchange process. As mentioned earlier, either the individual chiral ligand or one complexed with a metal ion is bonded onto silica gel support. Therefore, in the case of the first type of CSP, the metal ion is used in the mobile phase no metal ion is required in the mobile phase in the latter case. 

The ligand-exchange process has been applied as a mobile-phase-additive technique for enantioseparations. It involves the formation of a dissociable diastereoisomeric complex between a homochiral additive and a heterochiral solute about a central metal ion (Fig. 28). The mobile phase contains both the homochiral ligand and the metal ion as additive components. These species probably exist as the fully complexed species with at least two molecules of the homochiral... 

In order to characterize the ligand exchange process more quantitatively, studies have been made of the kinetics of water exchange for a wide variety of metal ions... 

A very complicated quantum mechanical model for computation of the energies of metal ion hydration has been used which, in addition to the coordination sphere, included two hydration shells, with limited success. There were attempts to apply molecular modeling for a better understanding of the ligand exchange processes. Success has been achieved in applying the ab ini-... 

Silica Polymei Metal Ion Interactions in Solution. The reaction of metal ions with polymeric sihcate species in solution may be viewed as an ion-exchange process. Consequently, it might be expected that sihcate species acting as ligands would exhibit a range of reactivities toward cations in solution (59). Sihca gel forms complexes with multivalent metal ions in a manner that indicates a correlation between the ligand properties of the surface Si-OH groups and metal ion hydrolysis (60,61). For Cu +, Fe +, Cd +, and Pb +,... 

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

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