Transition metal complexes rate constants

Figure 16.10 illustrates the results from application of ISM for the self-exchange rates in transition-metal complexes. These are at least as good as those provided by TM-1, but can also treat the anomalous case of the Co(OH2)6 system. This reveals that ISM provides an alternative view for electron-transfer self-exchange reactions in transition-metal complexes. It is a view not in terms of solvent- and solute-driven reactions, as in the formalism of Marcus, but entirely in terms of solute-driven reactions. The relevant factors are force constants, the sum of the equilibrium bond lengths of oxidised and reduced species and the electronic properties of the same bonds (bond orders). 

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... 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

A quantitative determination of such matrix elements (to be elaborated below) is of crucial importance because it not only allows an absolute evaluation of the desired rate constants but also helps to reveal the qualitative aspects of the mechanism. In particular, questions regarding the magnitude of electronic transmission factors and the relative importance of ligands and metal ions in facilitating electron exchange between transition metal complexes can be assessed from a knowledge of... 

An important feature to emerge from the comparisons in Table 2 is that variations in the electronic coupling term play a relatively small role in dictating the magnitudes of self-exchange rate constants for outer-sphere reactions, at least for transition metal complexes. Even for reactions... 

The rate of elementary reactions of certain transition-metal complexes, such as insertions or substitutions, can be controlled by the substituents at the metal center. In favorable cases, usually in families of closely related systems, these substituents can affect the reactivities and the chemical shifts of the transition metal nuclei in a similar, parallel fashion, resulting in an apparent correlation of both properties. Modem DFT methods can reproduce these findings, provided that changes in rate constants are reflected in corresponding trends in activation barriers or BDEs on the potential energy surface. 

The mechanistic analysis of the kinetics of electron transfer processes Involving transition metal complexes in solution continues to stimulate intense theoretical activity (1-17). In terms of the conventional transition state expression for the rate constant for activated electron transfer,... 

That the kinetically derived relative adsorption constants, Kab, decrease with the numbers of alkyl substituents is surprising because alkyl substituents increase the basicity of the benzene ring and stabilize Tl -arene transition metal complexes. The directly measured adsorption coefficients of benzene, toluene, p-xylene and mesitylene on a cobalt catalyst at 89 °C do increase with the number of methyl groups and the rates of hydrogenation decrease in that order. A consensus regarding the significance of the kinetically determined adsorption constants has not been reached. ... 

Some reactions are difficult to study directly because the required instrumentation is not available or the changes in standard physical properties (light absorption, conductivity etc.) typically used in kinetic measurements are too small to be useful. Competition kinetics can provide important information in such cases. In some situations, the chemistry itself makes direct measurement inconvenient or even impossible. This is the case, for example, in studies of slow reactions of free radicals. Because of the ever-present radical-depleting second-order decomposition reactions, slow reactions of free radicals with added substrates are possible only at very low, steady-state radical concentrations. The standard methods of radical generation (pulse radiolysis and flash photolysis) are not useful in such cases, because they require micromolar levels of radicals for a measurable signal. The self-reactions usually have k > 10 M s , so that the competing reactions must have a pseudo-first-order rate constant of lO s or higher (or equivalent, if conditions are not pseudo-first order) to be observed. Competition experiments, on the other hand, can handle much lower rate constants, as described later for some reactions of C(CH3)20H radicals with transition metal complexes. 

The kinetic approach to the analysis of photoprocesses has been summarized. Kinetic data are seldom sufficient for evaluation of all the primary rate constants. Nonetheless, it has been possible by combining photophysical and photochemical results to determine some of the rate constants in a variety of transition metal complexes. 

Fig. 5. Second order rate constants for the complex formation of the trivalent lanthanide ions and some d i-ions of transition metals with murexide as a function of reciprocal ionic radius (values from ref. 10)...

Rate constant data for several homonuclear electron transfer reactions involving transition metal complex ions in water are summarized in table 7.1. The striking feature of the results is that the rate constants vary over a very wide range from a low 2x 10 s to a high of 4 x lO M s. Since these... 

In comparing alkaline earth-for-transition metal exchange with transition metal-for-transition metal exchange, two important differences are apparent, Because of the larger stability constants of transition metal complexes, as compared with alkaline earth metal complexes, the disjunctive mechanism is less favorable and the adjunctive exchange pathway predominates. Also, as shown in Table 4, the rate constants for adjunctive exchange are slower for transition metal complexes than for alkaline earth complexes. 

Simple bimolecular electron transfer reactions can occur between radicals and nonradical species, and they can also occur between radicals and transition metal complexes. The number of reactions in this category is quite large, and they are notable for the wide range of reported rate constants. A sampling of these reactions, selected for their reversibility, is shown in Table 9.8. 

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