Metal alkyl hydrides

Metal alkyl hydrides 15 (R2A1H)2 or 3 (R = Me, Et), R2AlH(NMe3) Sources for growth of Al-based III-V alloys and A1 CVD... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

The complex TpPtMeH2 was synthesized by reacting TpPtMe(CO) with water (66). While it is stable towards reductive elimination of methane at 55 °C, deuterium incorporation from methanol-c/4 solvent occurs rapidly into the hydride positions and subsequently, more slowly, into the methyl position (Scheme 15). The scrambling into the methyl position has been attributed to reversible formation of a methane complex which does not lose methane under the reaction conditions (75,76). Similar scrambling reactions have been observed for other metal alkyl hydrides at temperatures below those where alkane reductive elimination becomes dominant (77-84). This includes examples of scrambling without methane loss at elevated temperature (78). 

Three years ago, while we were considering possible reasons for the general inability of transition metals to insert into and activate C-H bonds, our attention turned to the question of the instability of transition metal alkyl hydride complexes. We have listed the few alkyl hydride complexes of which we are aware (i) (one additional case (2) recently came to our attention) as well as some of the only slightly more numerous cases of substituted alkyl hydrides stabilized by chelation (3). In contrast, there are enormous numbers of polyalkyls (4, 5) and poly hydrides (6). While rarity does not logically imply instability, it does suggest it, so we considered possible mechanistic explanations for the assumed rapid decomposition of ci -MLn(R)(H) relative to cis-MLnR2 and cis-MLnH2. We have focused on octahedral complexes since they are both more important and more numerous. 

M—H bond energies are approximately constant and negligible difference in AS, the thermodynamic preference of metal aryl hydride complexes and free alkane over metal alkyl hydride complexes and free aromatic substrate suggests that the ABDE for M Ar versus M—R is greater than the ABDE for Ar—H versus R—H (Chart 11.6). 

Metal hydrides, which normally involve strongly electropositive metals such as sodium, lithium, boron, and aluminium, form hydrides like lithium hydride. Metal alkyl hydrides or metal aryl hydrides are elaborate molecules called complexes including HIr(CO)(CH3)-(P(C,H5)3)2C1. 

The thermodynamic driving force of olefin insertion into a metal-hydrogen bond is, of course, equal and with opposite sign to the thermodynamic barrier that prevents /3-hydride elimination from the metal alkyl hydride compounds, that is, the reverse of reaction (91) (R = H see Scheme 1). This decomposition mechanism will thus be more favorable for middle and late transition complexes, where metal-hydrogen bonds are much stronger than metal-alkyl bonds. 

In 1982-1983, three research groups (R. Bergman, W. Graham and W. Jones) independently reported the intermolecular oxidative addition of a C-H bond of alkanes on iP and Rh to give the corresponding metal-alkyl-hydride complexes. 

The protonation of a metal-alkyl complex usually gives a very instable metal-alkyl-hydride immediately leading to the reductive elimination of the alkane via the metal-o alkane species. The 16-electron intermediate, formed after loss of the alkane ligand, traps a solvent molecule as a ligand to complete its coordination sphere to 18 electrons ... 

The reductive elimination from metal-alkyl-hydride complexes L MR(H) (M = Ir, R = Cy or M = Rh, R = Ph) giving the alkane or arene occurs with inverse kinetic isotope effects which were taken into account by a preequilibrium with the a-alkane or r -arene complex. These reactions represent the microscopic reverse of oxidative addition of an alkane or arene C-Fl bond on electron-rich 16-electron metal centers... 

We also know from Chap. 3 that reductive elimination in 18-electron metal-alkyl-hydride complexes yielding an alkane and lower oxidation-state complexes is favorable for the first line of the transition metals if this lower oxidation state is accessible. 

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