Oxidation-reduction reactions between metal complexes

Oxidation-Reduction Reactions Between Complexes of Different Metals... 

We believe that catalysis occurs by formation of a complex between acetaldehyde, peracetic acid, and the metal ion in the 3+ oxidation state. The metal ion could be acting as a superacid as for peracetic acid decomposition, although oxidation-reduction reactions within the complex cannot be ruled out. Here again, we have found a disturbing lack of catalytic activity of other trivalent metals (aluminum, iron, and chromium). Simple acid catalysis is not as effective as proved when using p-toluenesulfonic acid and acetyl borate. This indicates that at least more than one coordination position is needed to obtain a complex of the proper configuration. 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

Most oxidation reactions are between specific metal cations or metal oxy-anions and cations. The problem that arises when applying oxidation-reduction reactions to soils is that all soils contain a complex mixture of oxidizable and reducible cations, anions, and organic matter, which means that it is impossible to determine which is being titrated. An exception to this is the oxidation of organic matter where an oxidation-reduction titration is routinely carried out. Organic matter determination will be discussed in Section 10.3. 

There are few reported data for the rates of electron transfer between the large complexes of these ligands. The rates are very large, and for the iron group metals NMR studies only allow a lower limit of 10 1 mole sec to be set (200, 224, 473, 474). The exchange between the tris complexes of Co(II) and Co(III) is found to catalyze ligand exchange for Co(III) (230) it has also been studied in nonaqueous media (504). Because of their convenient analytical properties, however, bipyridyl and phenanthroline complexes have been extensively examined in their oxidation reduction reactions. 

I really like to call my interest the study of oxidation-reduction (redox) reactions, and insofar as such reactions between metal ions are concerned, there was little prior work when I began my own. Werner had encountered redox reactions in the coirrse of his research and, in fact, made good use of them in the preparative procedures he developed. I m not sure that he thought much about the fundamental difference between them and the reactions he was primarily interested in, which were substitution reactions of metal complexes (coordination complexes). Many of the basic ideas underlying redox reactions were developed in the study of nonmetal chemistry. For example, my mentor at Berkeley became an expert in the redox chemistry of halogenates, after working with Professor Luther in Germany in the first part of this century. 

Oxidation-reduction reactions of transition metal complexes, like all redox reactions, involve the transfer of an electron from one species to another—in this case, from one complex to another. The two molecules may be connected by a common ligand through which the electron is transferred, in which case the reaction is called a bridging or inner-sphere reaction, or the exchange may occur between two separate coordination spheres in a nonbridging or outer-sphere reaction. 

The possibility of using C02 for the synthesis of fine chemicals that are now derived from petroleum has prompted efforts to obtain a broader understanding of the coordination chemistry of CO2 during the past 20 years.1-21 Carbon dioxide utilization will inevitably center on metal complexes and their ability to bind C02. In the past decade, many C02—metal complexes have been prepared and the ligand has demonstrated a remarkable variety of coordination modes in its complexes. The sections below outline the synthesis, characterization by X-ray crystallography and IR spectroscopy, and some characteristic reactions of these compounds. Also discussed are C02 insertion reactions into M—X bonds and oxidative coupling reactions between C02 and unsaturated substrates which occur at some metal centers. Finally, a profile of the research on catalytic reductions of C02 is provided. Where possible, references are made to reviews rather than to the primary literature. 

Perhaps the criteria which lead most directly to a conclusion are those which were applied in the first unambiguous demonstration of this kind of mechanism for reactions between metal ion complexes. When Co(NH3)5CP" reacts with Cr " (aq) in acidic solution, the products are Co " (aq), NH4+ and Cr(H20)5CF", the latter being formed virtually quantitatively. In the absence of further information, the identification of products would by no means provide proof of mechanism. The additional information which is pertinent to the issue follows (a) the oxidizing agent, Co(NH3)5CP", undergoes aquation much less rapidly than it does reduction by Cr (aq) ... 

In contrast to redox reactions involving organic species, reactions between metal ions and their complexes tend to be more straightforward and readily interpretable. One reason such reactions are easier to interpret is that the thermodynamic driving force for the reaction is usually well known, and multiple pathways to multiple products are much less common. In the following section we will discuss the results of a collection of studies of both Ce(III) oxidation and Ce(IV) reduction reactions by transition metals and their complexes. 

In this chapter the reactions between metal ions in a high oxidation state and inorganic and organic substrates are discussed in detail. Many mechanistic data have been derived from these investigations, and it is now clear that many of these reactions take place via an inner-sphere mechanism with, in some cases, evidence for the formation of well-characterised transient intermediates. Inner-sphere complex formation is more likely to take place where neutral or negatively charged substrates are involved rather than with cationic reductants, and three cases of the mechanism ... 

The preparation of many metal complexes often involves an accompanying oxidation-reduction reaction. For the thousands of cobalt(III) complexes that have been prepared, the starting material was almost always some cobalt(II) salt. This is because the usual oxidation state of cobalt in its simple salts is 2. The oxidation state of 3 becomes the stable form only when cobalt is coordinated to certain types of ligands (Section 5.2). Furthermore, it is convenient to start with salts of cobalt(n) because cobalt(ii) complexes undergo substitution reactions very rapidly, whereas reactions of cobalt(m) complexes are very slow (Section 6.4). The preparation of cobalt(ni) complexes, therefore, proceeds by a fast reaction between cobalt(ll) and the ligand to form a cobalt(ll) complex which is then oxidized to the corresponding cobalt(ni) complex. For example, reaction (19) is presumed to involve first the formation of [Co(NH3)6] ... 

It is now clear that complexes of transition metals in low oxidation states can act as bases. Proton NMR spectroscopy has been used to survey the protonation of a variety of transition metal complexes (54, 55). The entire idea of oxidative addition is based on the conceptual removal of electrons from such a basic metal center. The relationship between acid-base and oxidation-reduction reactions is particularly evident in cases where a proton becomes attached to a basic metal nucleus causing a two-unit increase in the formal oxidation state of the metal. Once coordinated, the proton is considered a hydride. Indeed the hydrogen often has hydridic character, reacting with another proton to yield hydrogen gas and the metal with an increase of two charge units. 

Chemical reactions between biochemical compounds are enhanced by biological catalysts called enzymes, which consist mostly or entirely of globular proteins. In many cases a cofactor is needed to combine with an otherwise inactive protein to produce the catalytically active enzyme complex. The two distinct varieties of cofactors are coenzymes, which are complex organic molecules, and metal ions. Enzymes catalyze six major classes of reactions 1) Oxidoreductases (oxidation-reduction reactions), 2) Transferases (transfer of functional groups), 3) Hydrolases (hydrolysis reactions), 4) Lyases (addition to double bonds, 5) Isomerases (isomerization reactions) and 6) Ligases (formation of bonds with ATP (adenosine triphosphate) cleavage) [1]. 

Many reactions are known to proceed by a free radical process. This type of reaction may be considered as something between types (1) and (2). For metal complexes such reactions are often classified as oxidation-reduction reactions. For example, in the last equation above, Br is reduced to Br while Co (II) is oxidized to Co (III). 

In volume 7 reactions of metallic salts, complexes and organometallic compounds are covered. Isomerisation and group transfer reactions of inert metal complexes and certain organometallics (not involving a change in oxidation state) are considered first, followed by oxidation-reduction processes (a) between different valency states of the same metallic element (b) between salts of different... 

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