Metal-bound hydride

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

Abstraction of one of the metal-bound hydrides from complex 5a provides the cationic iridium(lll) complex 28, which is an efficient precatalyst for alkyl halide reduction in the presence of EtsSiH (Equation 12.11) [31]. 

In the M-R bonded intermediate an a-elimination is not possible, whereas a P-elimination produces a metal hydride. This is energetically unfavorable compared to the oxy-bonded intermediate partly because of the lower M-H bond strength compared to MO-H (see section on thermodynamics). Thus, because of kinetics (lack of decomposition pathways) and thermodynamics (energetics), the metal bound M-R intermediate is less reactive on the surface than die M-O-R intermediate. 

CO formation on copper electrodes appears to be accompanied by hydride formation as well [103]. In Sch. 3, the surface bound CO is reduced by a hydride transfer reaction to form a formyl species as shown in step 2. There are precedents in organometallic chemistry for late transition metal hydrides reducing bound CO [105-109]. Protonation of the adsorbed formyl in step 3 results in the formation of a hydroxy carbene species [110, 111]. This hydroxycarbene species could be considered to be an adsorbed and rearranged form of formaldehyde, and the reduction of formaldehyde at a copper electrode has been reported to form hydrocarbons [102]. However, reduction of... 

Carbon-Hydrogen Bond Activation) with the metal bound hydride (M-H) to give a structure of the type M-H- -H-A, which leads to transfer of the proton to the hydride, at least initially. Likewise, coordinated H2 is often the site of highest kinetic acidity. ... 

Previous catalytic pathways for hydrogenation have involved transfer of a metal-bound hydride to the substrate containing the double bond, which is also bonded to the metal. This... 

X-ray diffraction is a standard method employed by chemists to determine the structure of new complexes. In the case of transition metal hydride and dihydrogen complexes, precise location of metal-bound hydrogen atoms by X-ray diffraction is very difficult. Superior structural information is provided by neutron diffraction, but the requirement for large well-formed single crystals has limited this method to a small subset of the known complexes.118... 

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