Metal-hydrogen bonds

Step 1 Hydrogen molecules react with metal atoms at the catalyst surface The relatively strong hydrogen-hydrogen c bond IS broken and replaced by two weak metal-hydrogen bonds... 

Hydrogen gas chemisorbs on the surface of many metals in an important step for many catalytic reactions. A method for estimating the heat of hydrogen chemisorption on transition metals has been developed (67). These values and metal—hydrogen bond energies for 21 transition metals are available (67). 

The principle of this method depends on the formation of a reversible diastereomeric complex between amino acid enantiomers and chiral addends, by coordination to metal, hydrogen bonding, or ion—ion mutual action, in the presence of metal ion if necessary. L-Proline (60), T.-phenylalanine (61),... 

The evidence is that the thermolytic route does not involve radicals but the photochemical one does. A dissociative mechanism for the thermolytic route is indicated by its inhibition by added phosphine it is likely that once a phosphine group has dissociated, a metal-hydrogen bond is formed, with generation of a coordinated alkene (Figure 3.58). 

Insertion of Silylenes into Metal-Hydrogen Bonds... 

Another very interesting reaction involving insertion of dicyclopentadienyl-stannylene into metal-hydrogen bond with displacement of its ligands has been described by J. G. Noltes et al.16S). The resulting product was identified by X-ray structural analysis. 

Preliminary investigations show that the principle of oxidative addition can be extended to compounds with transition metal hydrogen bonds. Thus, in the reaction of 1 with hydridopentacarbonylmanganese, the corresponding addition product can be isolated in high yield [29Si NMR 8 = 42.8 ppm, J(SiH) = 157.7 Hz IR v(SiH) = 2200 cnri, v(CO) = 1970, 1955 cnr1] [5],... 

The addition of several ligands to the metal system can reverse relative metal methyl and metal hydrogen bond dissociation energies. For example, reaction 5 is observed to be a last exothermic process,... 

The substitution of trialkylphosphine for carbon monoxide also makes the metal-hydrogen bond more hydridic in character and results in increased reduction of the initially formed aldehyde to alcohol. Slaugh and Mullineaux (52) compared Co2(CO)g and [Co2(CO)8 + 2PBu3], each at reaction conditions of 150°C, 500 psi, H2/CO I.0, for the hydroformylation of 1-pentene. The products consisted of hexyl aldehydes and hexyl alcohols in the ratios of 95 5 and 30 70, respectively. In a negative aspect of the reaction, they observed 23% hydrogenation of alkene to alkane at a reaction temperature of 195°C with the phosphine-modified catalyst. Tucci (54) reported less alkane formation (4-5%) under more favorable reaction conditions (I60°C, H2/CO 1.2, 1 hour reaction time). 

To the extent that mechanistic similarities exist, it is of interest to examine several crucial transformations in catalytic CO reduction and to see whether the organoactinide carbonylation results contribute to a better understanding of what may be occurring. The insertion of CO into a surface metal-hydrogen bond to produce a formyl (eq.(18)) has been discussed at length... 

Because of the lower metal-carbon and metal-hydrogen bond strength, organolead hydrides are particularly unstable species and represent the least stable of those of the group 14 elements. Triorganolead hydrides are obtained at low temperatures by reduction of the halides with LiAlH4 (equation 45), but they decompose at 0 °C. 

Scheme 17.3 Mechanisms of C02 insertion into a metal-hydrogen bond. L represents a potentially dissociable ligand. Ancillary ligands are not shown.

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Metal-hydrogen bonds generally give a high field signal in the 1H NMR spectrum in the region 15—35 r as illustrated by the data on tetranuclear clusters, which is sum-... 

Metal-Hydrogen Bond Enthalpy Contributions. Metal Carbonyl... 

An important implication of the values of D (M-CH2R) concerns metal-hydrogen bonds which are formed by both a-elimination... 

There is almost no reliable information about the strength of metal-hydrogen bonds in organometallic compounds. Earlier (Section 2.2.) it was established that M-H bonds formed by (spontaneous) -elimination from metal alkyls should be stronger that their precursor, but this is an unsatisfactory and imprecise position. The dissociation... 

The enthalpy of adsorption of hydrogen onto evaporated metal films, (M-H) has been measured for a few metals (Table 21b). Attempts112 have been made to calculate 2 (M-H) = 2 (M-H) — Z)(H-H), where if (M-H), the enthalpy of the metal-hydrogen bond, is given approximately by the equation due to Pauling. 

A mechanism which has been offered to explain these and other data is shown in the following scheme (s. p. 176). The initial step corresponds to the metallation of a metal-hydrogen bond. The rate of metallation of a haloalkene (which is essentially an electrophilic attack by the metal on the bond) depends on the electron density in the C—H bond. Obviously electron-withdrawing substituents such as halogen atoms would be expected to reduce the rate of metallation. We can rationalize the data for the reactions on platinum (100) by assuming that a halogen atom directly attached to the carbon of a C-H bond reduces the rate of metallation to a negligible value. 

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