Metal-hydrogen bonds cleavage reactions

From Metal-Hydrogen Bond Cleavage Reactions (J-4)... 

The metal-hydrogen and metal-carbon bond cleavage reactions have also been exploited in some instances (Section 2.10.2). 

Basically, the cleavage of the C-O bond takes place through two mechanisms. The first mechanism (the hydrogenolytic cleavage) is a hydrogen-assisted bond-cleavage reaction, whereas the second mechanism is an ionic insertion of the metal into the C-O bond. The characteristic features of the two mechanisms are summarized in Table 4.1. 

This established C-H chemistry in molecular coordination compounds should have its formal analog in metal surface chemistry. Carbon-hydrogen bond-cleavage is a common reaction at metal surfaces but it does not comprise mechanistically the simple collision of a hydrocarbon such as methane with a metal surface vide infra). Methane neither reacts with nor is chemisorbed on a metal surface under moderate conditions. However, propylene does chemisorb, and then C-H bonds may be broken, depending upon the nature of the metal surface. Generally, the sequence at the surface comprises first the formation of a coordinate bond between a surface metal atom(s) and some functionality in the... 

Silicon-hydrogen bonds also add to dinuclear complexes in reactions that are formally one-electron oxidative additions, or simply metal-metal bond cleavage reactions. The reaction of dicobalt octacarbonyl with an excess of silane leads to silylcobalt carbonyls (equation 13)2 3. Similar reactions involving Mn-Mn29, Re-Re30, Fe-Fe31, Ru-Ru32,... 

Further chemical evidence for the polarization of the metal-hydrogen bonds as M —H comes from cleavage reactions with aqueous acids, alcohols or phenols to produce H2, or with halogen (Br2 or I2) to give hydrogen halide, although these reactions are sometimes not quantitative. Some examples are ... 

M-C interaction in the product as well as reactant state results in weaker interaction between reacting hydrocarbon fragments, so that barriers for the reaction in both directions increase. This in line with the observed lower rate of hydrocarbon hydrogenolysis observed for Pt as compared with Ni. On Ni the rate of C-H activation is lowered more than that for the CH -CHy bond cleavage reaction. Because of the stronger metal-carbon bond of Pt than Ni, the rate of methane formation by recombination of adsorbed hydrogen with adsorbed CH3 will be lower on the former. 

Ti -vinyl [Ti2-C(CH2Ph)CH2] or hydride complexes is often followed by chemical steps, and examples of redox induced proton or hydrogen elimination (involving heterolytic or homolytic N-H, C-H or metal-H bond cleavage), nucleophilic attack or isomerization reactions have been presented. 

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