Hydride complexes metal-hydrogen bond

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

M—H bond dissociation energies, 1, 287 photochemistry, 1, 251 single crystal neutron diffraction, 1, 578 stability toward disproportionation, 1, 301 Metal—hydrogen bonds bond dissociation energy in 1,2-dichloroethane, 1, 289 stable metal hydrides in acetonitrile, 1, 287 thermochemical cycle, 1, 286 in THF and dichloromethane, 1, 289 olefin insertion thermodynamics, 1, 629 in Zr(IV) bis-Cp complexes, 4, 878 Metal—hydrogen hydricity data, 1, 292... 

Relaxation studies have also been used to estimate metal-hydrogen bond distances in rhenium and manganese hydride complexes (Gusev et al., 1993). 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Figure 2.8 Top The relationship between insertion of an alkene into a metal-hydrogen bond and the reverse /3-ehmination reaction for a rhodium complex. Bottom /3-elimination leading to the formation of a metal hydride and release of a polymer molecule with an alkene end group.

Phase B nearly always requires the dissociation of a ligand from ihc coordination sphere of the metal hydride complex formed in situ to produce a vacant coordination site, followed by coordination and insertion of the hydrocarbon into the reactive metal- hydrogen bond, as in following example an K... 

The insertion ofCOj into a transition metal-hydrogen bond produces a formate complex whicii reacts with water yielding formic acid and a transition metal-hydroxy Intermediate. In the presence of hydrogen, the hydroxy complex is reconverted into the primary metal hydride, with concomitant formation of water,... 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

The chemistry of metalloborane compounds is vast, although most are not prepared by oxidative addition of the B—H bond to a metal complex neither do they contain metal-hydrogen bonds. The synthesis of metalloborane hydrides involves many routes, the mechanisms of which are obscure. Reactions between borohydrides and transition-metal complexes involve hydride transfer without the formation of a metal-boron bond (see 1.10.7). 

Insertion of organic carbonyl compounds into metal-hydrogen bonds is important in organic synthesis. The reductions of esters, ketones, and aldehydes by main group hydrides and complex hydrides are of particular importance, but since the primary products of insertion are not characterized, they are not considered here . ... 

Carbon monoxide insertions into metal-hydrogen bonds have been elusive. The first direct formation of a metal-coordinated formyl group from a metal-hydride complex and carbon monoxide was observed with the hydride of octaethylporphyrinatorhodium(III), which reacts as follows with carbon monoxide at atmospheric pressure in benzene ... 

The many reactions that involve insertion of alkenes or alkynes into metal-carbon or metal-hydrogen bonds provide further examples of hypercoordination of carbon atoms during reactions. For example, an alkene may coordinate to the coordinatively unsaturated metal atom of a metal hydride complex prior to inserting into the metal-hydrogen bond [Eq. (1.9)] ... 

The reaction of transition metal hydrides and metal alkyls with CO2 frequently results in the formation of metal formates and carboxylates via an insertion of CO2 into a metal hydride or metal carbon bond. Step 2 of Scheme 1 (15-19). In some instances, the mechanism for this reaction has been investigated in detail. It has been found that the reaction can proceed by either a dissociative mechanism to produce a coordinatively unsaturated metal hydride as an intermediate, or it can occur by an associative mechanism (20-25). Thus, the metal hydride shown in Scheme 1 may or may not be required to be coordinatively unsaturated. Organometallic and metal phosphine complexes are again the two classes of complexes most commonly involved in CO2 insertions into metal hydrogen bonds (15-19). 

The first known complex in this class, frans-bis(triethylphosphine)-platinum chlorohydride, was prepared by Chatt and co-workers (86). In the light of other hydrides known at that time, in particular the carbonyl hydrides of iron and cobalt and bis(7r-cyclopentadienyl)rhenium hydride, the great stability of this complex was quite unique. It rapidly became clear that tertiary phosphine ligands were markedly effective in stabilizing the metal-hydrogen bond, and ligands such as PhjP, EtjP, o-CaH PlVfe, ... 

Complex hydrides of rhodium, Li4RhH4, and Li4RhH6 have been described (75). They were prepared by treatment of rhodium metal powder with lithium hydride at 600°C. The presence of transition metal-hydrogen bonds in either the latter complexes or those of Weichselfelder has yet to be unequivocally shown. Recent studies using a time-of-flight mass spectrometer have identified TiH4 and chlorohydride derivatives (29). 

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