Transition organometallic hydrides

Towards Catalytic Relevance Bimetallic Activation of Acyl Ligands and Transition Organometallic Hydrides as Reducing Agents... 

Transition organometallic hydride complexes CpFe(C0)PPh3(H), CpFe(Ph2PCH2CH2PPh2)H, and HFeCCO) " also reacted with a-alkoxy-ethylidene complexes 10,22 (64), but two pathways were observed. 

Ojima, I. Eguchi, M. Tzamarioudaki, M. Transition Metal Hydrides Hydrocarboxylation, Hydroformylation, and Asymmetric Hydrogenation. In Wilkinson, G. Stone, F. B. A. Abel, E. W., Eds., Comprehensive Organometallic Chemistry 2, Vol. 12, Pergamon, Oxford, 1995, Chapter 2. 

Thermodynamic data on the acidity of organometallic hydrides should help identify situations where apparent reactions of acidic transition-metal hydrides actually result from their conjugate bases. A case in which both species can react but give different products (as was pointed out by Prof. Espenson three years ago (18)) is the addition of hydridocobaloximes, HCo(dmgH)2B, to olefins with electron-withdrawing substituents... 

CO formation on copper electrodes appears to be accompanied by hydride formation as well [103]. In Sch. 3, the surface bound CO is reduced by a hydride transfer reaction to form a formyl species as shown in step 2. There are precedents in organometallic chemistry for late transition metal hydrides reducing bound CO [105-109]. Protonation of the adsorbed formyl in step 3 results in the formation of a hydroxy carbene species [110, 111]. This hydroxycarbene species could be considered to be an adsorbed and rearranged form of formaldehyde, and the reduction of formaldehyde at a copper electrode has been reported to form hydrocarbons [102]. However, reduction of... 

P. B. Armentrout, and L. S. Sunderlin, Gas-Phase Organometallic Chemistry of Transition Metal Hydrides, in Transition Metal Hydrides (Ed. A. Dedieu, VCH Publishers, New York, 1992, Chap. 1.4). 

Asymmetric Synthesis by Homogeneous Catalysis Carbonyl Complexes of the Transition Metals Hydride Complexes of the Transition Metals Organic Synthesis using Transition Metal Complexes Containing tt-Bonded Ligands Polynuclear Organometallic Cluster Complexes. 

Transition-metal hydrides react with nitrogen compounds to form N—H bond-containing organometallic products. The [HFe3(CO),J cluster anion reacts with nitriles to form a coordinated RCNH species, along with the RCHN-coordinated isomer ... 

Cocondensation of metal-atom vapors and organic ligands yields transition-metal hydrides as well as other organometallics (see Table 1). The bulk metal is vaporized at a controlled rate by resistance heating, laser heating or electron bombardment under vacuum [<10 torr (<0.1 Pa)]. The atoms condense either on the cooled walls (< — 100°C) of the vacuum chamber with an excess of the organic compound or into a solution of the compound in an inert solvent. Yields of hydride often are based on the amount of metal evaporated. 

Substrates covered are listed in the index page. Most investigators have worked with the complex hydrides of boron and aluminum therefore reactions with these form the principal part of the discussion. Some work with other organometallic hydrides and complex hydrides of some transition metals is also described. 

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 hydride ion (H-) is an efficient small ligand in organometallic chemistry. The first transition metal hydrides were prepared using the Hieber base reaction, exemplified in (6.50). The hydroxide adds to the carbon of one CO ligand to produce an intermediate that rapidly loses carbon dioxide, leaving the hydride ion to occupy the coordination site. 

The fields of organometallic chemistry and transition metal hydrides have grown together and are inextricably bound. A detailed coverage of the chemistry relating these two fields would be out of place here however, a variety of examples are considered below that show the importance of transition metal hydrides in the field of catalysis and organometallic chemistry. 

There are only limited pKj, scales for transition metal hydride complexes compared to the extensive scales for organic and inorganic acids, despite the fact that hydrides often mediate organometallic reactions these were reviewed in 1991 [42]. Most of these complexes contain carbonyl and/or phosphine ligands and are usually insoluble in water. The for about 20 neutral hydrido-carbonyl... 

The observation of this intermediate, which is in the equilibrium with the dihydrogen bonded complex 2 and the free cation 3 (Scheme 2), greatly contributes to the knowledge of the detailed mechanism of the proton transfer process to transition metal hydrides and is in accordance with the concept of proton transfer to metal atoms in organometallic complexes [19c, 21a, 24a]. 

Oxidative addition of transition metal-hydride and transition metal-carhon bonds to zero-valent transition metal complexes provides convenient method for preparation of homo- and heterodinuclear organometallic complexes. Oxidative addition of iron-hydride to zero-valent platinum complex giving Fe-Pt heterodinuclear complexes was demonstrated hy the reaction of HFe[Si(OMe)3](CO)3(/c -dppe) with zero-valent platinum complex such as Pt(C2H4)3 or Pt( 1,5-cod)2 giving eventually heterodinuclear ethyl or cyclooctenyl complex (Scheme 3.86) [175]. The resulting heterodinuclear structure is stahihzed hy the bridging dppe ligand and the siloxo moiety. 

The broad reactivity pattern displayed by transition metal hydrides is of special relevance in catalytic reactions and has led to a great deal of theoretical studics. The organometallic chemistry of hydridopaUadium complexes has been reviewed. The involvement of hydridopaUadium species is prevalent in Pd-catalyzed reactions. Their precise roles in catalytic hydrogenation (Sect. VII), Tsuji-Trost (Sect. V.2.1), and other related reactions are covered in detail in later sections. Only a brief introduction to their general reactivity of mechanistic relevance will be presented here. 

Transition metal hydrides constitute an important class of compounds with a relevant role in coordination and organometallic chemistry, namely as intermediates in catalysis [1], However, their electrochemical behaviour has not yet been sufficiently investigated, in spite of their expected ready activation by electrochemical methods [2-6], e,g, via the electrogeneration of reactive (paramagnetic) species. 

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