Stopped-flow metal-ligand reactions

A number of complexation reactions of inert-metal complexes with oxo ligands may occur by substitution at the more labile tetrahedral anionic center. The reactions of [CrfHjOlg] " and [Cr(NHj)5(H20)] + with arsenate have been reported. The reaction of the hexa-aqua complex has been measured by stopped flow methods the reaction of the penta-ammine is too fast at 25 °C to quantify with this technique. The substitution mechanism involves three parallel pathways, coupled by labile protonic equilibria involving the As(V) species as shown in Eq. (30). On the basis of the negative activation entropies and the dramatic dependence of the rate on the Cr(III) ligand , an associative mechanism is suggested. 

The aquated iron(III) ion is an oxidant. Reaction with reducing ligands probably proceeds through complexing. Rapid scan spectrophotometry of the Fe(III)-cysteine system shows a transient blue Fe(lII)-cysteine complex and formation of Fe(II) and cystine. The reduction of Fe(lII) by hydroquinone, in concentrated solution has been probed by stopped-flow linked to x-ray absorption spectrometry. The changing charge on the iron is thereby assessed. In the reaction of Fe(III) with a number of reducing transition metal ions M in acid, the rate law... 

On the coordination chemistry side, ligand substitution on metal complexes in ILs has attracted quite some interest. This is mainly due to the fact that both spectroscopic and catalytic properties are strongly governed by the nature of the ligands and the stability of their bond to a metal center. Begel et al. have studied the role of different ILs on ligand substitution reactions on [Pt(terpy)Cl]+ (terpy = 2,2 6, 2"-terpyridine) with thiourea with stopped-flow techniques. The substitution kinetics show similar trends if compared to conventional solvents with similar polarities. Moreover, much like in conventional solvents, the authors find an associative character of the substitution reaction [205], These results are essentially supporting an earlier study by Weber et al., who found the same behavior [206],... 

In the UV-visible spectra of organometallic compounds the spectral bands generally are large and less specific absorbances, due either to the transition metal or to the ligands in the compounds. So optical spectroscopy is used to study kinetic processes, to follow fast reactions (stopped-flow method), or to determine equilibrium constants. UV-visible spectra of the organometallic compounds are fre-... 

The results reported by Geiger et al. (1985) in a study of Ce(IV) oxidation of Co(II)-, Co(III)- and Ni(II)-tetrasulfophthalocyanine complexes provide evidence that the primary step is the formation of a ligand radical, rather than participation of the metal ion in the redox reaction. The combination of stopped-flow spectrophotometric and ESR data demonstrated that the Co(III)- and Ni(II)-phthalocyanine radicals disproportionate with regeneration of the original compound and ligand degradation products. In contrast, the Co(II) complex reaction proceeds via the formation of a radical and subsequent rapid reaction to produce the parent compound. 

Chattopadhyay et al [Ch 76a, Ch 76b, Ch 77] used a spectrophotometric stopped flow technique to study the kinetics of formation and dissociation of a large number of nickel complexes, with a metal to ligand ratio of 1 1, in various non-aqueous solvents. When their findings were compared with the results obtained earlier in investigations of analogous complexes of other transition metal ions [Ch 73, Ch 74], it was established that in most of the systems the above reaction follows an /d mechanism. The step determining the reaction rate was found to be the exchange between the solvent bound in the coordination sphere of the metal and the... 

Relatively few rate and mechanistic studies on tetrahedral complexes have been reported. These complexes are less common than octahedral complexes and their substitution reactions are frequently so fast that their study requires techniques such as stopped flow, temperature jump, or nuclear magnetic resonance. That the reactions are fast indicates that associative processes are occurring, since the activation energy for a reaction will be reduced when an incoming group can assist in the cleavage of a metal—ligand bond. 

Binary, ternary, and even quaternary mixtures of transition metals can be resolved by ligand substitution on the other hand, metal-substitution reactions permit the resolution of mixtures of alkaline earth metals. These determinations usually involve photometric detection thanks to the chromogenic reagents used also, signal readings are usually processed by using the logarithmic extrapolation method and stopped-flow instrumentation is required on account of the typically short half-lives of the reactions involved. 

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