Catalytic reactions electron counting

Reductive elimination is simply the reverse reaction of oxidative addition the formal valence state of the metal is reduced by two (or one in a bimetallic reaction), and the total electron count of the complex is reduced by two. While oxidative addition can also be observed for main group elements, this reaction is more typical of the transition elements in particular the electronegative, noble metals. In a catalytic cycle the two reactions always occur pair-wise. In one step the oxidative addition occurs, followed for example by insertion reactions, and then the cycle is completed by a reductive elimination of the product. 

The reductive elimination eventually releases the newly formed organic product in a concerted mechanism. In the course of this process, the electron count is reduced by two. Iron has a great tendency for coordinative saturation, which in general does not favor processes such as ligand dissociation and reductive elimination. This aspect represents a potential limiting factor for catalytic reactions using iron. 

Finally, many complexes that participate in homogeneous catalytic reactions have electron counts less than 16. This is especially true for high-oxidation-state early-transition-metal complexes such as (C2H5)TiCl3, Ti(OPr )4, etc. Cat-alytically active, late-transition-element complexes with electron counts less than sixteen are also known. An important example is RhCl(PPh3)2, a 14-electron complex that plays a crucial role in homogeneous hydrogenation reactions (see Section 7.3.1). 

Odd-electron metal clusters are frequently much more labile than are clusters which satisfy the electron counting rules. Catalytic substitution can be effected by one-electron reduction (or in some cases oxidation). 2 qqjg sequence of reactions leading to catalytic substitution is shown in Scheme 1. This scheme for catalytic substitution depends upon the substituted product having a less favorable reduction potential than the starting cluster. 

Oxidative addition (OA) and its reverse-reaction counterpart, reductive elimination (RE), play important roles as key steps in catalytic cycles (see Chapter 9) and in synthetic transformations (see Chapter 12). In the broadest sense, OA involves the attachment of two groups X-Y to a metal complex of relatively low oxidation state. This produces a new complex with an oxidation state two units higher than before, an increase in coordination number of two, and an electron count two higher than present in the starting material. Equation 7.24 outlines the essential changes present in an OA (and, of course, in the reverse direction, RE). 

In the oxidative addition of an A-B bond to a metal, new M-A and M-B bonds are formed as the A-B bond is cleaved (Eq. 2.1). The reverse reaction, reductive elimination, leads to the extrusion of an A-B molecule from a precursor M(A)(B) complex this is often the product forming step in a catalytic reaction. In the oxidative direction, we break the A-B bond and form an M-A and an M-B. Since A and B are always considered as le X-type (anionic) ligands, the oxidation state, the electron count, and coordination number of the metal all increase by two units during the reaction. The change of +2 in the formal oxidation state gives the reaction the name oxidative addition. These terms as well as the conceptual basis of organometallic chemistry are discussed in a previous work [4]. 

Each step of the hydroformylation cycle may be categorized according to its characteristic type of organometallic reaction. The cobalt-containing intermediates in this cycle alternate between 18- and 16-electron species. The 18-electron species react to formally reduce their electron count by 2 (by ligand dissociation, 1,2 insertion of coordinated alkene, alkyl migration, or reductive elimination), whereas the 16-electron species can increase their formal electron count (by coordination of alkene or CO or by oxidative addition). The catalytic activity is in large part a consequence of the capability of the metal to react by way of a variety of 18- and 16-electron intermediates. 

Oxidative addition reactions (and their reverse, reductive eliminations) are among the most important elementary transformations in organometallic chemistry and also play a key role in many stoichiometric and catalytic processes. Oxidative additions commonly involve the addition of a neutral molecule (X-Y) to a single metal center (M), resulting in the formation of new M-X and M-Y bonds and an increase by two units in the metal s oxidation state, electron count, and coordination number (Equation (5)). Although oxidative additions and reductive eliminations are in principle reversible reactions, the position of the equilibrium, which is governed by the overall thermodynamics of the species involved (i.e., the relative strengths of the bonds broken and formed), is often completely shifted to one of the sides. 

Of these, the first four typically play the biggest roles in the major catalytic cycles discussed in this chapter. Electron counting plays an important role in identifying some of these reaction types (especially oxidative addition and reductive elimination), and the ionic method of electron counting will be used in this chapter. 

Many of the reaction mechanisms are important to catalysis and many organometallic compounds are used widely as homogenous catalysts. Some of the reactions include catalytic alkane polymerization, oxidative additions, and reductive elimination. Although these are very specific reactions with different mechanisms used to describe each, you should be able to understand them by ensuring the electron counting rules are obeyed. 

The first step in the Cativa process is the reaction between Mel and c -[Ir(CO)2l2]. However, the catalyst may also react with HI and this step initiates a water gas shift reaction that competes with the main catalytic cycle, (a) What chemical is manufactured in the Cativa process Why is this product of industrial importance (b) Why is HI present in the system (c) Give an equation for the water gas shift reaction, and state conditions typically used in industry, (d) Figure 25.22 shows the competitive catalytic cycle described above. Suggest identities for species A, B, C and D. What type of reaction is the conversion of czj -[Ir(CO)2l2] to A What changes in iridium oxidation state occur on going around the catalytic cycle, and what is the electron count in each iridium complex ... 

Cataljdically active, late transition metal complexes with electron counts less than 16 are also known. Examples are RhCXPPhj), PdCPBUj ), and [PdtPhjCPBUj jBr] (see 2.29 and 2.36). These are 14-electron complexes and, as we will see later, take part in many homogeneous catalytic reactions. 

A general difficulty inherent in these defect-counting and attribution stages arises from the possibility of mutual reactions of complementary defects. Suppose a characteristic color is the result of a trapped electron and that a particular catalytic activity in the same solid results from a trapped hole. If the electron is released upon warming and annihilates the particular trapped hole, then the color and the catalytic activity will seem to be connected. More complex confusion can arise if one species of trapped electron can be annihilated at several kinds of trapped hole. Similar problems can arise with atomic defects, since vacancies and interstitials have an analogous complementary relationship. 

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