Dihydrides, theoretical studies

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Reductive elimination of from stable cis dihydride complexes can be photochemically promoted and undoubtedly M-H2 intermediates are involved. Theoretical studies of gas-phase reactions of Fe(CO)5 with OH indicate that H2... 

The oxidative addition of H2 is much more common than the reaction of alkane C—H bonds. Both thermodynamic and theoretical studies of oxidative addition of X—H bonds to metal complexes identify the relative weakness of M—C bonds as an important factor in the generally lower thermodynamic stability of alkyl hydrides relative to dihydrides or silyl hydrides. Halpern " identified a number of species where D(M—H) is about... 

In 2002, Sakaki and coworkers carried out a theoretical study on the mechanism of hydrogenation of CO2 catalyzed by the three-coordinate, 14e d monohydride model complex RhH(PH3)2 [13]. It was found that CO2 insertion into the Rh-H bond is almost barrierless. This result suggests that the 14e rhodium(I) hydride is very reactive toward CO2. The COj insertion leads to formation ofthe rhodium (I) formate intermediate Rh(PHg)2(0C(0)H). To the rhodium(I) formate intermediate, oxidative addition of H2, which leads to formation of a dihydride complex, requires a barrier of 7.3 kcal mol". Then the reductive elimination of formic acid in the dihydride complex via a five-membered ring transition state (1) was calculated to have a very small barrier of 1.9 kcal mol . In the o-bond metathesis pathway, a barrier of only 6.1 kcal mol" was calculated. The o-bond metathesis... 

In 2002, Sakaki also carried out theoretical studies on the mechanism of hydrogenation of CO2 catalyzed by the square pyramidal, 16e d dihydride complexes cfs-[RhH2(PH3)2(H20)] [13]. In the cfs-[RhH2(PH3)2(H20)] -catalyzed reaction, CO2 inserts into a Rh-H bond to give a formate intermediate with a barrier of 28.4kcal mol . Then the total barrier calculated for o-bond metathesis followed by release of formic acid was 5.9 kcal mol . ... 

The appreciation of H2 complexes was delayed by the notion that such complexes could not be stable relative to classical dihydrides. Even the theoretical basis for interaction of H2 and other o-bonds with a metal was still undeveloped at the time of the initial discovery. Ironically, a computational paper by Saillard and Hoffmann7 in 1984 on the bonding of H2 and CH4 to metal fragments such as Cr(CO)s was published only shortly after our publication of the W-H2 complex (without mutual knowledge). Such interplay between theory and experiment has continued to be an extremely valuable synergistic relationship.3,4,8 The innate simplicity of H2 was attractive computationally, but the structure/bonding/dynamics of H2 complexes turned out to be extremely complex and led to extensive study (>300 computational papers). 

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