Solvent and Ligand Exchange

As indicated in Section 5.7.4.3, the effects of ion pairing on rates of dimethyl sulfoxide exchange with the [Co(NH3)5(dmso)] cation have been established.The small accelerating effects of ion pairing are listed in Table 5.14. Activation enthalpies and activation entropies for dimethylformamide exchange with [Co(NH3)5(dmf)] both decrease as the proportion of dimethylformamide increases in water + dimethylformamide solvent mixtures. These trends are discussed in terms of an interchange mechanism and the effects of interactions between free and coordinated dimethylformamide molecules on the outer-sphere association constant. 

The rate of exchange of nitrite with the 2-amino-2-methyl-3-butanoneoxime (LLH, 37) complex [Co(LL)(LLH)(N02)2] is independent of the concentration 

Substitution Reactions of Inert Metal Complexes—6 and Above 

Kinetics of exchange of acetylacetone with the complexes [M(acac 3], M = Co, Rh, Ru, and Cr, have been examined in solvent acetylacetone, over the temperature range 358-463 K. The order of decreasing rates is 

Activation parameters and deuterium isotope effects indicate that there is a difference of mechanism between [Co(acac)3] and the other three complexes. In all cases it is thought that the mechanism involves the generation of a monodentate acac intermediate, but this rate-determining step is thought to be dissociative for cobalt (AH = 36.4 kcal mol A5 positive) but associative for the other metals (27.4 A// 28.8 kcal mol A5 negative).The exchange of acetylacetone with [Co(acac)3] has also been studied in acetonitrile and in dimethyl sulfoxide and dioxan. In the latter investigation effects of added protons were studied by addition of chloroacetic acid. 

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Direct kinetic studies of water exchange on [Pd(H20)4]2+ and [Pt(H20)4]2+, the parameters for which appear in Table XIV (267-274), were first reported well after the chemistry outlined in the preceding paragraph had become established. Solvent and ligand exchange on... 

Quantum chemical methods are well established, accepted and of high potential for investigation of inorganic reaction mechanisms, especially if they can be applied as a fruitful interplay between theory and experiment. In the case of solvent exchange reactions their major deficiency is the limited possibility of including solvent effects. We demonstrated that with recent DFT-and ab initio methods, reaction mechanisms can be successfully explored. To obtain an idea about solvent effects, implicit solvent models can be used in the calculations, when their limitations are kept in mind. In future, more powerful computers will be available and will allow more sophisticated calculations to be performed. This will enable scientists to treat solvent molecules explicitly by ab initio molecular dynamics (e.g., Car-Parrinello simulations). The application of such methods will in turn complement the quantum chemical toolbox for the exploration of solvent and ligand exchange reactions. 

Table 2 Selected exchange rate constants and activation parameters for solvent and ligand exchange reactions on square-planar Pt complexes...

Solvent and ligand exchange kinetics 348 3.10. Oxidation of peroxocompounds ... 

The rates of solvent and ligand exchange of the lanthanides are in general quite rapid, being accessible only through rapid-kinetic techniques like stopped-flow. 

In chapter 102, Drs. K.L. Nash and J.C. Sullivan explore the chemical kinetics of solvent and ligand exchange in aqueous lanthanide solutions. These authors deal with redox reactions readily available only from the Ce(IV)/Ce(III) and the Eu(II)/Eu(III) couples among the lanthanides. A wealth of tabulated information on rate and equilibrium constants is provided in textual and tabular form. 

In summary, nonaqueous systems are less labile compared to aqueous systems, which facilitates the determination of their coordination numbers, and solvent and ligand exchange rate laws. 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

Attempts have been made to treat selective ion solvation in mixed solvents as ligand exchange reactions [36 e]. We express ion X (cation or anion) existing in solvent A by XA and in solvent B by XB,-. Here, nf=ri if the molecular size of A is very different from that of B or if A is unidentate and B is bidentate (e.g. PC and DME in the solvation of Li+). Otherwise, it is usual that n=n. Then, the ligand exchange reaction in the mixture of A and B will proceed as follows with the increase in the concentration of B ... 

Much more research has been carried out with polymers in which the coordinated metal atom is part of the chain backbone. Typically, the metal atoms are copper, nickel, and cobalt. Oxygen atoms or carbon atoms adjacent to the metal atom provide the electrons required for the coordinate bond.30 Polymers of this type are often rather intractable, for a variety of reasons. Specifically, insolubility can be a problem for species with moderate molecular weights. Also, coordination between chains can cause aggregation, and ligand-exchange reactions with small molecules such as solvents can cause chain scission. However, in some favorable cases, the intramolecular coordination is sufficiently strong for the polymer to be processed by the usual techniques such as spinning into fibers or extrusion into films.30... 

Cui XJ, Khlobystov AN, Chen XY et al (2009) Dynamic equilibria in solvent-mediated anion, cation and ligand exchange in transition-metal coordination polymers solid-state or recrystallization Chem Eur J 15 8861-8873... 

There are two potential side reactions that can derail the catalytic cycle shown in Scheme 10.6, namely, hydroboration of the olefin monomer by borane and ligand exchange reactions between the borane and cocatalysts containing aluminum alkyl groups. Fortunately, borane compounds containing B-H groups usually form stable dimers (Figure 10.1) that are unreactive toward olefins in typical olefin polymerization solvents (e.g., hexane, toluene). To eliminate the second process, however. 

First we must find a suitable solvent for the compound. Deuterated solvents are normally used, for the reasons outlined in Section 4.3.1. The NMR solvent has to dissolve suitable quantities of the sample, typically of the order of mg in 0.5 ml of solvent, but more will be needed for less sensitive nuclei. It must not react with the compound under investigation specific care has to be taken not to overlook exchange reactions between donor solvents and ligands in metal complexes, because the deuterated donor solvent molecules bound in the complex are not detectable by means of proton NMR. The required experimental temperature also limits the choice of solvent, which must of course be in its liquid range. 

Solvent Exchange, Racemization, Isomerization, and Ligand Exchange... 

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