Hydrogen carbonyl hydrides

Covalent. Formed by most of the non-metals and transition metals. This class includes such diverse compounds as methane, CH4 and iron carbonyl hydride, H2Fe(CO)4. In many compounds the hydrogen atoms act as bridges. Where there are more than one hydride sites there is often hydrogen exchange between the sites. Hydrogens may be inside metal clusters. 

Some reactions of carbonyl hydrides will be illustrated in Chapter 22. Such species are involved in catalytic processes in which metal carbonyls function as hydrogenation catalysts. Generally carbonyl hydrides are obtained by acidifying solutions containing the corresponding carbonylate anion or by the reactions of metal carbonyls with hydrogen. The following reactions illustrate these processes ... 

Some carbonyl hydrides, such as Cr2(CO)ml l, have the hydrogen atom in bridging positions as shown in the structure... 

Two hydrogen bridges are present in some carbonyl hydride complexes. This is illustrated by the structure of the [H2W2(CO)8]2 anion,... 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

Hydrogen forms an extensive series of compounds with the metal carbonyls, and the nature of the H bonding within these complexes has been a point of debate for a considerable period. Both the chemistry and structure of metal carbonyl hydride compounds have been exten-... 

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

Submitted by MICHAEL 1. BRUCE and MICHAEL L. WILLIAMS Checked by GUY LAVIGNE and TH RESE ARLIGUIEt This tetranuclear ruthenium carbonyl hydride was described on several occasions,5 but early preparations were usually contaminated with Ru3(CO)12, giving rise to suggestions of the existence of two isomeric forms. The situation was clarified by the work of Kaesz and coworkers,6 who discovered the direct route from Ru3(CO)12 and hydrogen, which is described below. The compound is often obtained from reactions between Ru3(CO)12 and substrates containing hydrogen (hydrocarbons, ethers, alcohols, water, etc.) and by acidification of anionic ruthenium cluster carbonyls.7... 

Sterically hindered alkenes can be hydrogenated at —50°C using triflic acid and a hydride donor.483 In addition to Et3SiH, transition-metal carbonyl hydrides such as HM(CO)3Cp (M = W, Mo, Os) and HMn(CO)5 (M = Mn, Re) are suitable hydride donors. Alkenes that form tertiary carbocation on protonation are hydrogenated in high yields (90-100%), whereas yields for styrenes are lower (50-60%). Alkynes are converted to the corresponding saturated hydrocarbon by using HW(CO)3Cp in combination with triflic acid.484... 

The conjugation process 1,4-diene — 1,3-diene goes by interconversion of 7r-Pd complexes I and III via the intermediate 7r-allyl-Pd hydride complex II and involves 1,3-hydrogen shifts. Conversion of cis,trans to the thermodynamically favored trans,trans-conjugated dienes occurs, but both of these isomers would be carboxylated the same way. CO insertion via a Pd carbonyl hydride (allyl or olefin) intermediate gives unsatu-... 

In contrast to the other carbonyl hydrides of vanadium, H3V(CO)3-(diars) is remarkably thermally stable and resistant to deprotonation. Even n-butyllithium fails in this regard, though it is possible to replace two of the hydrogens in the photochemical reaction... 

It is probably not correct to think of the hydrogen atoms as directly bound to the metal atom but to the CO group, thus Me—C = 0—H. This is indeed an unusual bond proportion with a twofold positive formal charge of the oxygen atom, but the hydrogen is readily replaceable by metals, while with acids the carbonyl hydride can be regenerated again from it. 

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