Oxidative cleavage metal hydride

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

M H (M = Si, Ge, Sn) bonds are reactive to undergo oxidative addition of transition metals, homolytic cleavage giving group 14 metal radicals, and so on. Some studies have been reported on the formation of M E (M = Si, Ge, Sn E = S, Se, Te) bonds by using the group 14 metal hydrides. 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Intermediate metal hydrides or methylmetals may decompose by the pathways shown in Fig. 6. Heterolytic cleavage giving CH3, H+, or CH3, H, or homolytic cleavage giving CH3- or H- will reduce the metal atom by 2, 0, or 1 electron(s), respectively. Present chemical evidence for reductive demethylation 40 44,48) or oxidative methylation 122, 131, 137)... 

Heterolytic cleavage. This leads to formation of a metal hydride with release of a proton (equation 1). The formal oxidation state of the metal does not change. This mode of hydrogen activation is common in hydrogenation by complexes of ruthenium(II). 

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