Reactions of Metal -Complexes

H. Taube (Stanford) mechanisms of electron transfer reactions of metal complexes. 

Mechanisms of substitution reactions of metal complexes. F. Basolo and R. G. Pearson, Adv. Inorg. Chem. Radiochem., 1961, 3, 1-89 (132). 

Application of molecular orbital theory to electron transfer reactions of metal complexes in solution. J. K. Burdett, Comments Inorg. Chem., 1981,1, 85-103 (7). 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

J. Lewis and R. S. Nyholm Structure and reactions of metal complexes of chelate olefin ligands, pp. 61-99 (37). 

As regards intimate mechanism, electron transfer reactions of metal complexes are of two basic types. These have become known as outer-sphere and inner-sphere (see Chapter 4, Volume 2). In principle, an outer-sphere process occurs with substitution-inert reactants whose coordination shells remain intact in... 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

In some cases, reactions of metal complexes, most often metal anions, with EX3 or ER3 lead to simple adduct formation (II in Scheme 1) rather than displacement of X or R. New structurally characterized complexes appearing in the past decade are found in Table 1. 

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

Major emphasis is placed on the reactions of metal complexes in solution undergoing either inner-sphere ligand substitution or electron transfer to and from the metal center. Such studies relate to the important selective role of metal catalysts in many areas of enzymatic, commercial, and modem synthetic chemistry. Clearly, this field has now matured to the point where basic theoretical considerations, although incomplete, can provide a logical framework for understanding the chemical reactivity of such systems and stimulate the investigation of (1) new and unique reaction pathways, (2) modified reagents, and (3) unorthodox matrices. 

Reactions involving nonmetallic species or nontraditional reactions of metal complexes (unusual oxidation states, reactions with different reaction partners, etc.) are less commonly studied but are becoming of increased interest as mechanistic inorganic chemistry has come of age. A... 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

A recently proposed semiclassical model, in which an electronic transmission coefficient and a nuclear tunneling factor are introduced as corrections to the classical activated-complex expression, is described. The nuclear tunneling corrections are shown to be important only at low temperatures or when the electron transfer is very exothermic. By contrast, corrections for nonadiabaticity may be significant for most outer-sphere reactions of metal complexes. The rate constants for the Fe(H20)6 +-Fe(H20)6 +> Ru(NH3)62+-Ru(NH3)63+ and Ru(bpy)32+-Ru(bpy)33+ electron exchange reactions predicted by the semiclassical model are in very good agreement with the observed values. The implications of the model for optically-induced electron transfer in mixed-valence systems are noted. 

An important conclusion that can be drawn from the above discussion is that most outer-sphere electron transfer reactions of metal complexes are, at best, marginally adiabatic and that the reaction will rapidly become nonadiabatic with increasing separation of the reactants. In view of these considerations, eq 11 can be integrated to give (50)... 

As we have seen, an area of major importance and of considerable interest is that of substitution reactions of metal complexes in aqueous, nonaqueous and organized assemblies (particularly micellar systems). The accumulation of a great deal of data on substitution in nickel(II) and cobalt(II) in solution (9) has failed to shake the dissociative mechanism for substitution and for these the statement "The mechanisms of formation reactions of solvated metal cations have also been settled, the majority taking place by the Eigen-Wilkins interchange mechanism or by understandable variants of it" (10) seems appropriate. Required, however, are more data for substitution in the other... 

The rate constant for one reaction may have to be correlated with the equilibrium constant not for that reaction but for a related one. Rate constants for reactions of metal complexes... 

Due to the numerous applications that have stimulated studies of interactions and reactions of metal complexes with DNA, we cannot cover all the aspects in this review. We will focus our attention on photoreactions of metal complexes with DNA. Although, obviously for carrying out photochemical reactions, it is essential to discuss the binding of these compounds to DNA. Dark reactions, on the other hand, will not be described. 

F. Reactions of Metal Complexes with 1,3-Diynes Giving Organic Products. 218... 

Reaction of metal complexes with S is a convenient method for directly introducing the ligand by substitution of other ligands. For example, NajS2 or polysulfide solutions can be used for this purpose (77, 107, 108, 120,146). 

A related approach to the preparation of highly dispersed supported bimetalhc catalysts involves the reaction of metal complexes with supported metal clusters or particles. The method is based on the idea that by careful choice of the metal complex and control of the reaction parameters it may be possible to cause the metal complex to react selectively with the supported metal but not with the support surface [13]. Because this approximation to the subject is the main focus of this chapter, it is thoroughly developed in the following sections. 

Mechanisms of Substitution Reactions of Metal Complexes Fred Basolo and Ralph 0. Pearson... 

Jhe reactions of metal complexes may be grouped into the following categories ... 

Here, it should be noted that some reactions of metal complexes also produce ECL. 

Reactions of metal complexes with suitable heterocarbonyl precursors constitute another route to heteroaldehyde and heteroketone complexes. Very often, these reactions involve the cleavage of a heterocyclic compound. 

The conclusions described in the previous section are inferred from a relatively small number of observations of spin-equilibrium dynamics. Nevertheless, they are internally self-consistent and also compatible with a much wider set of observations derived from studies of electron transfer reactions of metal complexes. For these reasons there is hope that they possess some generality and can be applied to other systems. 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

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