Metal complexation rates

This review is concerned with the quantitative aspects of metal-catalysed oxyradical reactions. As such one will find discussions of structures of metal complexes, rate constants and reduction potentials, not unlike our review of 1985 [34], Two areas related to the role of transition metals in radical chemistry and biology have been reviewed recently these are the metal-ion-catalysed oxidation of proteins [35] and the role of iron in oxygen-mediated toxicities [36]. These topics will not be discussed in detail in this review. Related to this work is a review on the role of transition metals in autoxidation reactions [37]. Additional information can be obtained from Afanas ev s two volumes on superoxide [38,39], This subject is also treated in a more general and less quantitative manner by Halliwell and Gutteridge [40],... 

Insertion reactions between alkynes and Ni(acac)(PPh3)(CH3) [e.g., reaction (b)] have been explored for a series of alkynes. Reactions were found to be first-order in alkyne and first-order in metal complexes. Rates of insertion follow the first order ... 

Because of their high negative normal redox potentials, Sm + and Yb + are able to reduce many ions. This property has not yet been used for preparative purposes but it has been exploited in mechanistic studies of electron transfer reactions involving transition metal complexes. Rate constants of reactions between Yb + ions (prepared by electrolytic reduction of Yb " ) and some Co " " complexes were measured (Christensen et al., 1970). More accurate kinetic measurements, with the pulse radiolysis technique, have been made (Faraggi et al., 1973) on the reactions of Yb + and Sm + with [Co(NH3)5Xp+ (X = F, Cl, Br, I, N3, NCS, OH, CN), [Co(NH3)d +, [Ru(NHj)5X]2+ (X = Cl, Br, I, OH) and [Ru H3)J +. The reactions were performed in deaerated aqueous solutions containing tert-butyl alcohol. 

Ultrasonic absorption is used in the investigation of fast reactions in solution. If a system is at equilibrium and the equilibrium is disturbed in a very short time (of the order of 10"seconds) then it takes a finite time for the system to recover its equilibrium condition. This is called a relaxation process. When a system in solution is caused to relax using ultrasonics, the relaxation lime of the equilibrium can be related to the attenuation of the sound wave. Relaxation times of 10" to 10 seconds have been measured using this method and the rates of formation of many mono-, di-and tripositive metal complexes with a range of anions have been determined. 

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

The electron transfer rates in biological systems differ from those between small transition metal complexes in solution because the electron transfer is generally long-range, often greater than 10 A [1]. For long-range transfer (the nonadiabatic limit), the rate constant is... 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

The donor-acceptor approach to solvent effects on the rates of redox reactions between different metal complexes, R. Schmid, Rev. Inorg. Chem., 1979,1,117-131 (48). 

The Marcus treatment applies to both inorganic and organic reactions, and has been particularly useful for ET reactions between metal complexes that adopt the outer-sphere mechanism. Because the coordination spheres of both participants remain intact in the transition state and products, the assumptions of the model are most often satisfied. To illustrate the treatment we shall consider a family of reactions involving partners with known EE rate constants. 

One of the commonest reactions in the chemistry of transition-metal complexes is the replacement of one ligand by another ligand (Fig. 9-3) - a so-called substitution reaction. These reactions proceed at a variety of rates, the half-lives of which may vary from several days for complexes of rhodium(iii) or cobalt(m) to about a microsecond with complexes of titanium(iii). 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Few quantitative data are available on the relative nucleophilicities of L toward various alkyl carbonyls. The rates of the reaction of CpMo(CO)3Me with L in toluene (Table II) decrease as a function of the latter reactant P( -Bu)3 > P( -OBu)j > PPhj > P(OPh)j, but the spread is relatively small (<8). The above order is that customarily observed for 8 2 reactions of low-valent transition metal complexes (J, 214). Interestingly, neither CpMo(CO)3Me nor CpFe(CO)2Me reacts with 1 or N, S, and As donor ligands 28, 79). This is in direct contrast to the insertion reactions of MeMn(CO)5 which manifest much less selectivity toward various L (see Section VI,B,C,D for details). 

Rates of Spin-State Transition for Solid Metal Complexes Based on the Line Shape... 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Another area of active research is the development of stable low molecular weight metal complexes, which could serve as SOD mimics. Fridovich has described a complex of mangsmese (III) with desferral, which can catalyse the dismutation of superoxide anion in vitro and can protect green algae against paraquat toxicity (Beyer and Fridovich, 1989). This manganese-desferral complex was evaluated in models of circulatory shock and also found to improve survival rate (de Garavilla etal., 1992). 

Preparation of mixed metal oxides - The sulfated metal oxides (zircoiua, titaiua and tin oxide) were synthesized using a two-step method. The first step is the hydroxylation of metal complexes. The second step is the sulfonation with H2SO4 followed by calcination in air at various temperatures, for 4 h, in a West 2050 oven, at the temperature rate of 240°C hSulfated zirconia Zr0Cl2.8H20 (50 g) was... 

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