Coordination transition metal hydride

If we now consider the CO/H2 system, then the catalytic process can be envisaged as a combination of (1) and (2) above, carbon monoxide being activated by coordination and hydrogen by addition. If we adopt the view that in a transition metal hydride the hydrogen is present as an anionic hydride ligand ( H ) (49) then, given the charge separation... 

In summary, many attempts have been made at achieving enantioselective reduction of ketones. Modified lithium aluminum hydride as well as the ox-azaborolidine approach have proved to be very successful. Asymmetric hydrogenation catalyzed by a chiral ligand-coordinated transition metal complex also gives good results. Figure 6-7 lists some of the most useful chiral compounds relevant to the enantioselective reduction of prochiral ketones, and interested readers may find the corresponding applications in a number of review articles.77,96,97... 

Hydrides of Pt(II) are the most numerous of any transition metal hydride group. In addition to the presence of the hydride ligand, the complexes invariably have a coordinated phosphine, and synthetic routes to these compounds using both hydridic and protonic reagents have been reported (I). The pure complexes are usually both air stable and kinetically inert. The purpose of this chapter is to show the diversity of hydrides that can be obtained from protonation reactions on zero-valent and di-valent triphenylphosphine platinum compounds, and to rationalize the type and nature of the product formed from the character of the acid HX. 

Main group hydrides are of particular interest in themselves but are relevant to coordination chemistry only to the extent that they are used to prepare transition metal hydrides or act as ligands. 

In coordinatively saturated metal hydrides, such as the HM(CO)s (M => Cr, Mo, W) derivatives, formation of the four-centered transition state for C02 insertion (Scheme 1) may proceed with or without CO loss and concomitant coordination ofC02 at the metal center. That is, C02 insertion may occur by means of dissociative (D) or dissociative interchange (Id) processes, or an associative interchange (Ia) process (47, 48). In either instance an acid-base interaction between the anionic hydride ligand and the electrophilic carbon center of carbon dioxide as represented in 6 may occur prior to formation of the four-centered transition state depicted in Scheme 1. An interaction of this type has been observed for these HM(CO)j derivatives with Lewis acids such as BH3 (49). 

Crystallographic studies of transition metal hydride complexes Stereochemistry of six-coordination Five-coordinate structures Stereochemistry of five-coordinate Co complexes Absolute stereochemistry of chelate complexes Stereochemistry of optically-active transition metal complexes Electron density distributions in inorganic compounds... 

Transition metal hydrides may show hydridic (H8-, Figure 2.16) or protic reactivity (H8+, Table 2.4). In addition, there are some characteristic reactions that fall outside this categorization, which involve coordination of the substrate to the metal centre, prior to combination with the hydride ligand (Figure 2.30). 

An attack of a nucleophilic transition metal hydride on coordinated CO has also been observed by Labinger and Wong, who treated carbonyls of Ct, Mo, W, Mn, Fe, Ku and Co with Cpi Nbllj. which is of a lower hydridic nature than the main group hydrides or group IV hydridic complexes (54]. Small amounts of C C3 hydrocarbons are formed. However, when the reaction of CpjNbH with Cr(CO)(( is carried out under H], only ethane t formed. Carbene intermediates are proposed to account for this selectivity (Scheme 14). 

The proposed reaction mechanism (see Scheme 4) is similar to that which is well established for olefin reduction, with the insertion of a meial-hydride bond into the carbonyl group coordinated to (he metal center. A mechanism in which reduction occurs by a simple nucIcopliiUc attack of the hydrogen atom of the transition metal hydride (which behaves like Blit or AlHa ) into the carbonyl group of the aldehyde, cannot be ignored ... 

Asymmetric Synthesis by Homogeneous Catalysis Carbonylation Processes by Homogeneous Catalysis Coordination Organometalhc Chemistry Principles Electronic Stmcture of Clusters Hydride Complexes of the Transition Metals Hydrides Sohd State Transition Metal Complexes Organic Synthesis using Transition Metal Complexes Containing 7t-Bonded Ligands Oxidation... 

SCF and CAS SCF calculations on mono and bimetallic transition metal hydride complexes are reported. The importance of including the non dynamical correlation elTects for the study of the cis-trans isomerism in dihydrido complexes and for the study of the CO insertion reaction into the metal hydride bond is stressed. The metal to metal hydrogen transfer in a class of bimetallic d — d hydride complexes is analyzed and the feasibility of the transfer discussed as a function of the coordination pattern around the two metal centers. 

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 ... 

Transition-metal hydride reactions can result in P—H bond formation. White phosphorus and (h -C5Hj)2MoH2 yield the metal-coordinated P2H2 ... 

Trialkylborohydride reducing agents differ from borohydride in their ability to transfer hydride directly to a carbonyl ligand without prior substitution in the coordination sphere. They are used to synthesize formyl complexes " . When formyl complexes lose CO and undergo hydride migration from the formyl ligand to the metal, a transition-metal hydride results. The process is formally similar to nucleophilic attack by [OH] on a carbonyl ligand, followed by loss of CO and formation of a transition-metal hydride. Examples of hydride syntheses via formyl complexes are ... 

Insertion of alkenes into transition metal hydrides has assumed great importance because of the key roles such reactions play in coordination catalysis and in organo-metallic synthesis. Such reactions are treated in Section 15. Despite the large number of... 

Coordination of an alkene to a coordinatively unsaturated transition metal hydride is an essential step in homogeneous hydrogenation. In those cases where it has been possible to follow the reaction stereochemistry, the transfer of hydride from the metal to the coordinated alkene has been observed to proceed in a stereospecifically syn fashion [Eq. (6.156)] ... 

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