Coordination compounds oxidation-reduction reaction

Reactions of coordination complexes can be conveniently divided into substitution reactions at the metal center, oxidation-reduction reactions, and reactions of the ligands that do not change the attachments to the metal center. Reactions that include more elaborate rearrangements of ligand structures are more often observed in organometallic compounds description of these reactions is given in Chapter 14. 

The reactions described to this point are either substitution reactions or oxidation-reduction reactions. Other reactions are primarily those of the ligands in these reactions, coordination to the metal changes the ligand properties sufficiently to change the rate of a reaction or to make possible a reaction that would otherwise not take place. Such reactions are important for many different types of compounds and many different conditions. Chapter 14 describes such reactions for organometallic compounds and Chapter 16 describes some reactions important in biochemistry. In this chapter, we describe only a few examples of these reactions the interested reader can find many more examples in the references cited. 

An electron transfer (ET) reaction is defined here as an oxidation-reduction reaction that occurs between two coordination compounds. The compounds may be the same species, but where the metals have different oxidation states (a selfexchan reaction), or are completely different species (a cross-reaction). An example of a self-exchange reaction is given in Equation (17.41), where Co is an isotopically labeled cobalt a cross-reaction is given by Equation (17.42). 

This reaction is a variation of the hydroformylation reaction. Transmetallation of Rh(I)(acac) with the alkylmercury(I) compound gives ClHg(acac) and an alkylrhodium(I) compound. Oxidative addition of H2 gives a Rh(III) compound, and coordination and insertion of CO gives the acylrhodium(IH) compound. Reductive elimination then gives the product and regenerates Rh(I) — but as a Rh-H, not as Rh(acac). 

Complexes involving oxime ligands display a variety of reactivity modes that lead to unusual types of chemical compounds. As far as the oxime chemistry of platinum is concerned, these complexes are involved in facile deprotonation of the OH group with formation of oximato complexes, reduction of Pt(IV) species, Pt(II)-assisted reactions with coordinated allene," alkylation by ketones, oxime-ligand-supported stabilization of Pt(III)—Pt(III) compounds, oxidative conversion into rare nitrosoalkane platinum(II) species, and coupling with organocyanamides. ... 

A soft catalyst is one of Pearson s soft acids if it is a compound of a metal it will be a complex of one of the later members of Group VIII (i.e., Ru, Os, Rh, Ir, Pd, or Pt), usually in a low oxidation state. These soft catalysts undergo 2-electron oxidation reductions, and the substrate reacts while coordinated to the metal. Such reactions are also called heterolytic, as there is heterolytic splitting of the C—H bond [C—H — Cr + H+ (or C+ + H )]. 

Most reactions of coordination compounds can be classified as either substitutions or oxidation-reductions. The classic book by Basolo and Pearson19 discusses both types in detail. The oxidation-reductions can occur either by simple electron transfer or by atom transfer. Taube s work on the reduction of cobalt(IJI) complexes by Cr11 is especially important in this regard. Among the many reactions which he has studied, the best known, perhaps, is20... 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

Primary photoreactions leading to net oxidation or reduction reactions of coordination compounds are well known and are often the result of decay paths accessible only from CT states. A number of coordination compounds yield photoelectron production in solution, the Ru(2,2 -bipyridine)3+ ion has been shown to be an electron donor from its electronically excited state, and photoreduction of several metal complexes has been studied in detail. Discussion of these three areas should reveal most of the important principles associated with photoredox and CT state chemistry. 

An organometallic radical may undergo several different types of reactions Scheme 3 illustrates some different reactions of CpM(CO)3 radicals (21) including (i) dimerization (ii) halide abstraction from an alkyl hahde or metal halide (iii) hydrogen atom abstraction from metal hydrides (iv) electron-transfer reduction (v) electron-transfer oxidation and ligand addition (vi) electron-transfer induced disproportionation (see Electron Transfer in Coordination Compounds). 

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