Thermodynamics of oxidative addition

Finally, this analysis of the data in Table 6.2 shows that the oxidative addition of the C-H bond of methane and the C-C bond of ethane to this iridium compoimd is thermodynamically unfavorable. Althougji die reaction is predicted to be sligjitly favored enthalpicaUy, the free energy is positive because of the large positive TAS term for a process that generates one product from two reactants. The entropy for a reaction of this t37pe is usually dominated by translational entropy, and this value is t5 icaUy about -35 eu. Near room temperature, TAS will then equal about -10 kcal/mol, and the free energy for addition of methane and ethane to this Ir(I) complex is positive. 

The remainder of this chapter presents the reactions of nonpolar reagents with a range of metals. The reactions of dihydrogen are presented first the reactions of silanes are presented second because they often react like dihydrogen. The additions of C-H bonds are presented third examples of intramolecular additions of C-H bonds are presented before intermolecular examples. This chapter closes with a discussion of the oxidative addition of C-C bonds. 

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Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

Thus it is important to take into accoimt the thermodynamics of oxide formation and any additional electrochemical... 

The catalytic cycle is common with the single carbonylation of aryl hafides in the steps of oxidative addition of aryl halide to a Pd(0) species and the subsequent CO insertion process. Since successive CO insertion into the acylpalladium bond is not thermodynamically favorable, incorporation of another CO molecule takes the route of coordination of the CO molecule to the acylpalladium(II) intermediate. Attack by a nucleophile such as secondary amine, alcohol, and water in the presence of an appropriate base, including the amine itself, gives bis acyl type intermediate which reductively efiminates a-keto acid derivatives [131]. 

The oxidative addition of H2 is much more common than the reaction of alkane C—H bonds. Both thermodynamic and theoretical studies of oxidative addition of X—H bonds to metal complexes identify the relative weakness of M—C bonds as an important factor in the generally lower thermodynamic stability of alkyl hydrides relative to dihydrides or silyl hydrides. Halpern " identified a number of species where D(M—H) is about... 

Reductive elimination is the reverse of oxidative addition. This class of reaction forms products from the coupling of two covalent ligands at a single transition metal center (Equation 8.1) or two ligands from two different metal centers (Equations 8.2 and 8.3). This reaction is the product-forming step of many catalytic processes. Because oxidative addition and reductive elimination are the same reaction occurring in opposite directiorrs, the formation of products from oxidative addition or reductive elimination depends on the thermodynamics of the two processes. In some cases, equilibria between the two reactions have been observed directly, but more often the thermodynamics favor addition or elimination to a sufficient extent that high yields of either the addition or elimination products are obtained. Mormation on the thermodynamics for several types of oxidative addition and reductive elimination were provided in Table 6.2. 

Many factors control the rates of reductive elimination reactions (Equation 8.4). In many cases, the effects of the steric and electronic properties of the metal complex on the rate of reductive elimination are the opposite of the effects of these properties on the rate of oxidative addition because the effects originate from thermodynamic factors. The steric and electronic properties of the metal ttiat thermodynamically favor oxidative addition must thermodynamically disfavor the opposite reductive elimination reaction. 

The knowledge of enthalpies of dissociations for M —H, M —R, M —C(0)R, and Y —C(0)R (Y = H, R) bonds allows the estimation of enthalpies of various organome-tallic reactions (Table 4.4). This table shows that the following reactions are thermodynamically favorable oxidative addition of hydrogen, CO insertion into the M —R bond, olefin insertion into the M —H bond, and olefin insertion into the M —R bond. Oxidative addition of aldehydes to the transition metal complexes is possible. However, due to the near-zero value of the enthalpy of this reaction, oxidative addition of aldehydes does not occur easily in the above-mentioned processes. Thermodynamically unfavorable are oxidative addition of the unstrained C —C bonds, oxidative addition of C — H bonds, and CO insertion into the M — H bonds. 

A measure of the thermodynamic stability of various Ir(III) compounds has been obtained by Yoneda and Blake, who have measured enthalpies of oxidative addition of I2, HI, and various alkyl and acyl iodides to... 

Oxidative addition offers a direct method to cleave a covalent bond. Although a wide variety of bonds, such as C-1 and C-Br, are known to facilely undergo oxidative addition reactions to low-valent transition metal complexes, examples of oxidative addition of C—C single bonds are far more rare. The scarcity is in part associated with the thermodynamic stability of C—C bonds. Whereas oxidative addition of C-Br and C-I bonds to low-valent metals is thermodynamically favored in general, that of a C—C single bond is often thermodynamically disfavored. 

Braunschweig H, Damme A. Thermodynamic control of oxidative addition and reductive elimination processes in us-bis(dimethoxyhoryl)-his(tricyclohexylphosphine)-platinum(II). Chem Commun. 2013 49 5216-5218. 

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

Extensive studies have been carried out by Giggins and Pettit and by Vasantasree and Hocking on a range of nickel chromium alloys with up to 50% alloying addition. Generally the principles outlined above can be used to interpret the experimental observations, where the thermodynamics of the reaction are a major factor determining the rate of attack, depending upon whether oxide or sulphide is the stable phase. 

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