Hydrides metal—carbon bonds

The propagation centers also react with the inhibitors inevitably present in the reaction medium. The interaction with coordination inhibitors may stabilize the transition metal-carbon bond, as the elimination of the coordinative insufficiency of the transition metal ion makes it impossible for the metal-carbon bond to rupture through the mechanism of the /3-hydride shift. 

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

In the presence of H2, perhydrocarbyl surface complexes loose their ligands through the hydrogenolysis of their metal carbon bonds to generate putative hydride complexes, which further react with the neighbouring surface ligands, the adjacent siloxane bridges (Eqs. 8-9) [46,47]. 

Because the two metal-carbon pi bonds now extend into both dimensions perpendicular to the axis of the metal-carbon bond, the residual metal-hydride bonds are all constrained to lie essentially orthogonal to the M—C axis (i.e., in the nodal hollows of the pi-bonding dxz and d, orbitals). The optimized structures, as shown in Fig. 4.16, all reveal this common structural tendency, with near-perpendicular (91-96°) H—M—C bond angles in all cases. 

The metal hydride bond is stronger than a metal carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Alkene insertion into a metal hydride is thermodynamically allowed and often reversible. 

We conclude that our working hypothesis is valid, and that when ML4 is a sufficiently unstable fragment, both dinuclear elimination and metal-carbon bond homolysis can occur instead of simple reductive elimination of R-R from ML RR. We conclude further that the involvement of a hydride ligand is necessary for dinuclear elimination from such ML RR. On that basis, we propose the above general mechanism to explain the instability of hydridoalkyls of this type. 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

Donor adducts of aluminum and gallium trihydride were the subject of considerable interest in the late 1960s and early 1970s.1 Thin-film deposition and microelectronic device fabrication has been the driving force for the recent resurgence of synthetic and theoretical interest in these adducts of alane and gallane.24 This is directly attributable to their utility as low-temperature, relatively stable precursors for both conventional and laser-assisted CVD,59 and has resulted in the commercial availability of at least one adduct of alane. The absence of direct metal-carbon bonds in adducts of metal hydrides can minimize the formation of deleterious carbonaceous material during applications of CVD techniques, in contrast to some metal alkyl species.10, 11... 

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. 

The hydrocarboxylation can take place by insertion of the alkene into a metal-hydride bond followed by CO insertion and finally reaction of the acyl complex with solvent as illustrated in equation (36). Alternatively, a transition metal-carboxylate complex can be generated initially. Insertion of the alkene into the metal-carbon bond of this carboxylate complex followed by cleavage of the metal-carbon bond by solvent completes the addition, as shown in equation (37). Both sequences provide the same product. 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

The reduction behaviour of the alkylidene adduct of a cobalt-dithiolene complex (423) has been examined548 and the study has shown that, when the alkylidene-bridged structure (423) is reduced by one electron, it isomerizes rapidly and quantitatively to the ylide form (424). This represents the first example of reversible isomerization of the metal-carbon bond in a cobaltadithiolene complex. A surprising cis- to tra .s-dihydride isomerization which is unprecedented for 18-electron six-coordinate complexes has been observed549 in an octahedral iridium-c7.y-di hydride complex. 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

Thermodynamically the insertion of an alkene into a metal-hydride bond is much more favourable than the insertion of carbon monoxide into a metal-methyl bond. The latter reaction is more or less thermoneutral and the equilibrium constant is near unity under standard conditions. The metal-hydride bond is stronger than a metal-carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Insertion of alkenes is also a reversible process, but slightly more favourable than CO insertion. Formation of new CT bonds at the cost of the loss of the ji bond of the alkene during alkene hydrogenation etc., makes the overall processes of alkenes thermodynamically exothermic, especially for early transition metals. 

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