Reaction kinetics of coordination compounds

In the majority of chapters of this book the concern has been with an understanding of the properties of individual molecules, properties which are regarded as essentially time-independent. However, no less important are the chemical reactions of these molecules and, here, changes as a function of time are of the essence. This chapter is devoted to a review of our present understanding of some of the reaction types which are characteristic of coordination compounds. It is as well to recognize the complexity of the problem. Suppose we are interested in a reaction such as the aquation of an ion such as [Co(NH3)5Cl] , a much studied system  

In Section 2.1 the distinction between inert and labile complexes was encountered. Several attempts have been made to formalize this distinction, of which the most popular seems to be Taube s definition if no delay is noted in the substitution reaction under ordinary conditions (i.e. room temperature, ca. 0.1 M solutions) the system will be described as labile . However, for most chemists inert complexes are effectively those for which their reactions may be studied by classical techniques, such as monitoring the change in intensity of a visible or ultraviolet spectral peak with time. Such reactions are half complete in about one minute or longer at 25 °C for 

The top spectrum is the low-temperature, no exchange, limit. The bottom is the high-temperature, rapid exchange, limit. The central spectrum was recorded at ca. 200 X. Those immediately below and above were recorded at ca. 190 X and 210 X, respectively. This particular example arises from two proton environments within a molecule becoming rotationally equivalent with increase in temperature. For the case described in the text the two low-temperature peaks would be of different areas and the high temperature at the area-weighted average position. 

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To classify the varying rates of reaction (most commonly with regard to substitution) of coordination compounds, Henry Taube, who received the 1983 Nobel Prize in chemistry for his work in the kinetics of coordination compounds, suggested the terms and inert. If we consider a 0.1 Af aqueous solution, a lahk coordination compound is one that under these circumstances has a half-life of less than a minute. (Recall that half-life is the amount of time required for the concentration of the reactant to decrease to half its initial concentration.) An inert coordination compound, on the other hand, is one with a half-life greater than a minute. 

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

Formulation of the detailed mechanisms of decomposition of coordination compounds are likely to remain difficult. Reliable kinetic and supporting observations are not easily obtained where several initiating reactions are possible and subsequent chemical changes may occur, before the first-formed product has left the crystallite of reactant. 

Espenson, J. H. (1995). Chemical Kinetics and Reaction Mechanisms, 2nd ed. McGraw-Hill, New York. A book on chemical kinetics, much of which is devoted to reactions of coordination compounds. Highly recommended. 

Wilkins, R. G. (1991). Kinetics and Mechanisms of Reactions of Transition Metal Complexes. VCH Publishers, New York. Contains a wealth of information on reactions of coordination compounds. 

Several classes of coordination compound undergo several successive, reversible one-electron-transfer reactions. These comprise a so-called electron-transfer chain or series .8 Cyclic voltammetry is particularly useful for recognizing such behaviour and an example is illustrated by Figure 2. This shows the four members of the electron-transfer chain [Fe4S4(SPh)4]"-, n = 1-4.5 An electron-transfer series provides the coordination chemist with a means of examining the consequence of systematic addition (or removal) of electrons from a nominally fixed geometry thermodynamic, kinetic and spectroscopic relationships between members of a series can be explored.9... 

Electronic spectra may be used (as in organic chemistry) as fingerprints, and they are very important in kinetic studies. The change in the electronic spectrum of a reaction mixture as the reaction proceeds is often the best way of following its rate, and quite elaborate methods are available for measuring very fast reaction rates. However, the application which the reader is most likely to encounter in more advanced texts is in the area of coordination compounds of the transition elements, whose electronic spectra may yield information about structure and bonding. 

An excellent kinetics book that deals heavily with reactions of coordination compounds. 

The importance of substitution reactions cannot be overstated. Systematic investigations of coordination compound substitution kinetics, and mechanisms shed light on the electronic structure of compounds and on their interactions. Although formally taken up in Chapter 4, substitution is encountered in all chapters of this book. 

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