Transition metal hydrides carbonyl compounds

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

The state of the art of reductions with metal hydrides a decade ago was the subject of comprehensive reviews. A detailed survey of reductions of carbonyl compounds with alkali and alkaline earth metal hydrides, borane and derivatives, alane and derivatives, metal borohydrides, metal aluminohydrides, silanes, stannanes and transition metal hydrides was compiled. The properties, preparation and applications of each reagent were discussed together with methods for their determination, handling techniques... 

Ethyl rhenlumpentacarbonyl has been reacted with various metal hydrides in acetonitrile 158). The observed products were heterobimetallic compounds, although a solvated rheniumtetracarbonyl acyl complex was detected [Eq. (47)]. If the metal hydride is in excess, the rate-determining step is formation of the propionyl complex. The reaction was subsequently found to be first order in both the propionyl complex and the metal hydride. The second-order rate constants were measured and were found to be the reverse of the order of the acidities of the transition metal hydrides, which implies that the hydrides react as nucleophiles with the propionyl complex. In a separate experiment, [Re(COEt)(CO)j] was found to react with [Re(H)(CO)j] only after carbonyl dissociation, implying that the metal and not the acyl carbonyl is the site of nucleophilic attack by transition metal hydrides on acyl complexes. 

The olefinic bond of a,g-unsaturated carbonyl compounds can also be reduced by two transition metal hydrides, NaHFe2(CO)881 and NaHCr2(CO)10 82 Neither reagent reduces nitriles, ketones, aldehydes or non-conjugated carbon-carbon double bonds. 

The chemistry of transition metal hydride (TMH) complexes started back in 1931 with the discovery of H2Fe(CO) by Hieber and coworkers [1]. This compound indeed contains two single iron-hydrogen bonds. In subsequent years, related metal carbonyl hydrides appeared, such as HCo(CO) [2,3]. During the same period of time, the industrially important hydrofomylation process was discovered [4,5]. Back then, it was not clear that these two findings were directly related to each other. As we know nowadays, HCo(CO)4 plays an important role in the hydrofomylation process [6], and constitutes probably the first application of TMHs in catalytic processes. 

To the best of our knowledge, this is the first report of a dealkylation of tliis type mediated by a transition metal cluster carbonyl. The formation of this hydride could explain in part the large amount of amine 57 formed during the catalytic reactions. This compound has in fact been considered a key intermediate in the catalytic hydrogenation of nitrobenzene to aniline [97]. However, as has been discussed in Chapters 4 and 6, the participation of hydride derivatives in this type of reduction of the nitro to amino group is doubtful. 

Retaining the theme of metal carbonyl clusters, capping considerations in transition-metal clusters have been discussed with reference to [Sb2Co4(CX))] g( A-CX))], and [Bi2Co4(CO)jQ( i-CO)]" 28. An infrared spectroscopic study of the formation of carbonyl rhodium clusters on a rhodium electrode produced by oxidation reduction cycles in acidic solution 2 has also been published. Electrochemistry with ruthenium carbonyls >21 osmium carbonyls 2 jg also reported. Muon spin rotation in a metal-cluster carbonyl compound has been communicated and, lastly, a proton spin-lattice NMR relaxation study of hydride carbonyl clusters has been reported. This provides a method for determining distances involving hydrido ligands... 

Reductive aldol reaction of a,(5-unsaturated esters and enones with aldehyde mediated by a transition metal hydride complex and a hydride source, such as hydrosilane, is a versatile process to produce p-hydroxy carbonyl compounds (Scheme 15a) [21]. This reaction is thought to be an alternative transformation of Lewis acid-catalyzed Mukaiyama-type aldol reaction with silyl enol ethers or silyl ketene acetals (Scheme 15b). 

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. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Silicon and germanium hydrides react with cobalt, manganese and rhenium carbonyls affording complexes having a silicon (or germanium)-metal bond. These reactions, described previously for inactive compounds have been used in the synthesis of optically active silyl and germyl-transition metals ... 

Next to the cyclopropane formation, elimination represents the simplest type of a carbon-carbon bond formation in the homoenolates. Transition metal homoenolates readily eliminate a metal hydride unit to give a,p-unsaturated carbonyl compounds. Treatment of a mercurio ketone with palladium (II) chloride results in the formation of the enone presumably via a 3-palladio ketone (Eq. (24), Table 3) [8], The reaction can be carried out with catalytic amounts of palladium (II) by using CuCl2 as an oxidant. Isomerization of the initial exomethylene derivative to the more stable endo-olefin can efficiently be retarded by addition of triethylamine to the reaction mixture. 

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