Cobalt, ligand exchange rates

Stable cobalt (III) ion complexes with which ligand exchange does not occur were prepared by the scheme of equation (32) (Alexander and Busch, 1966 Buckingham et al., 1967a, b, c), and were isolated and characterized. Large rate enhancements in hydrolysis are observed in comparison with the uncomplexed esters for attack of Hj O at pH 1-4, but it is of greatest interest that the reactions are markedly catalysed by general bases. It has been observed that... 

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

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. 

Wilkins [314] that the oxygenation of cobalt(II) complexes is controlled by the ligand exchange process, i.e. the replacement of coordinated water (ligand) or solvent molecule by dioxygen. The rate... 

Lead.— Exchange reactions involving polyamine-A-polyacetate complexes of lead(ii) continue to be studied. An investigation of the substitution of lead(ii) by cobalt(n) has been reported, and there has been a re-interpretation of earlier data on ligand-exchange kinetics between [Pb (edta)] and R-(—)-pdta. Rates... 

Attempts have been made to separate steric and electronic factors governing ligand substitution in tetranuclear clusters of cobalt and iridium. For a series of isostructural complexes [Co4(CO)nL], the rate of dissociative loss of CO has been determined by measuring CO exchange rates. The ligand L is a phosphine or phosphite, and occupies an axial position as shown in structure 21. The relative rates of CO dissociation in heptane... 

The absence from this rate law of a term in the concentration of pyridine is explained by rapid conversion of the cobalt(n) complex into [Coii(dmgH)2(py)], followed by a rate-determining inner-sphere electron- and ligand-transfer step. Ligand-exchange reactions... 

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