Homolytic activation

A different class of catalyst is exemplified by HCo(CN)3-, formed by homolytic activation of H2 by Co(CN)5 ion. These catalysts are particularly effective for reducing activated alkenes, for example XCH=CH2 to XEt (where X = C02H, C02R, Ph, CN) and 1,3-dienes to monoenes. Radical intermediates have been implicated in these reactions.156... 

The formation of hydrides by activation of molecular hydrogen implies the transformation of H2 into MH ", MH or MH. The hydrogen activation can be either homolytic or heterolytic, and can occur at single or multiple metal centres. Examples of homolytic activation are given in Equations (1) and (2). 

The aqueous-phase dehydrogenation mechanism [24] was found to follow similar paths and relative ordering of the C-H and O-H activation steps as that found for the vapor-phase case [15,16,17], despite the fundamental differences that result from the heterolytic versus homolytic activation. The optimized structures [24,25] and reaction energies [24,25] (including vapor-phase reaction energies [15,16,17] in parentheses) for the aqueous phase methanol decomposition over Pt(l 11) are presented in Figure 19.5. 

Ever since their discovery, the intimate relationship between molecular hydrogen complexes and homolytic activation of a o bond through oxidative addition has been obvious and has allowed a deeper insight into this important process. Furthermore, new visions of heterolytic activation have recently emerged [10] proton transfer from a previously coordinated a bond to another ligand seems to play a role in heterolytic bond breaking reactions much more often than expected. Thus, o complexes do not seem only to be possible intermediates in oxidative addition processes, but are also becoming compounds with a chemistry of their own (Scheme 2). 

The presence of solution can dramatically alter the resulting chemistry at the solution-metal interface. This is clearly present even in the neat liquid phase dissociation processes alone. For example, the dissociation of acetic acid proceeds in the vapor phase at higher temperatures via a homolytic process that leads to the formation of CHsC02 and H free radical intermediates. This homolytic activation of the 0-H bond in acetic acid costs 440 kJ/mol[ l. The heterolytic activation of acetic acid to form CH3CO2- and H" " intermediates, however, is significantly more endothermic, costing -1-1532 kJ/mol. 

Hydrogenation catalysts add molecular hydrogen to the C=C group of an alkene to give an alkane. Three general types have been distinguished, according to the way each type activates H2. This can happen by (1) oxidative addition, (2) heterolytic activation, and (3) homolytic activation. 

Gontrary to the long time belief that a higher oxidation state iron(III) complex is inactive for the homolytic activation of the dormant G-X bond, the Fe(III) complexes alone were quite recently employed as the catalyst without any intentionally added reducing agents. ° ° The detailed mechanism of the polymerization only by Fe(III) is still unclear, although the... 

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