Cobalt complexes—continued kinetics

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

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... 

Chromium(ni) continues to be the second most widely studied metal ion after cobalt(m). The question of dissociative (/a) or associative (/a) mechanisms for substitutions at chromium(m) has been clarified somewhat by comparing data for the formation and dissociation of the [Cr(H20)5X] + and [Cr(NH8)5X] + ions (X = unidentate leaving anion). Recent studies indicate that an associative mechanism is important for aqua-chromium(m) complexes, but for the [Cr(NHa)6X] + ions a dissociative-interchange mechanism is favoured. A summary of kinetics and thermodynamic data for the formation and aquation of [Cr(NH3)5X] + ions is given in Table 12. 

Interest continues in the effects of micellar agents on rates of inorganic reactions. Aquation of oxalato-cobalt(m) and -chromium(iii) complexes is mentioned in a review of reversed micellar systems predicted and actual rates have been compared for a variety of reactions, including the mercury(n)-catalysed aquation of [CoX(NH3)g] + cations. Micellar effects on uncatalysed aquation of cis- and of tra j -[CoCl2(en)2]+ are small (as expected) but real. Other reactions for which the kinetic consequences of micelle or polyelectrolyte addition have been d cribed include complex formation from nickel(ii), the conversion of ammonium cyanate into urea, and the hydrolysis of pyrophosphate. ... 

Although cobalt(m) and chromimn(m) continue to be kinetically the most widely investigated inert metal ions, some progress has been made in recent years towards our understanding of the rarer metals. The continuing efforts of Taube and his associates are particularly noteworthy in the field of ruthenium chemistry, and several interesting publications from them dealing with aspects of ruthenium(n) and ruthenium(in) complexes have appeared recently. - The photochemical studies of... 

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