Unsaturated hydride complex

The coordinatively unsaturated hydride complex is proposed to be active site in the hydrogenation of olefins under mild conditions [24]. 

L = PMe2Ph. The hydrido complexes [WCp2(H)(L)] (L = PMeaPh or MeCN) are presumably formed by thermal or photoinduced cleavage of the substituted alkyl ligands (—CH2L) from species 46 followed by reaction of excess ligand with the coordinatively unsaturated hydride complex 47. 

For this reason, the unsaturated hydride complex appears to behave as an efficient hydrosilation catalyst. ... 

Conversion of 362 (TfO salt) to the coordinatively unsaturated hydride complex 372 (TfO salt) is achieved by treatment with HSiEts at RT. This product forms an adduct with acetonitrile at its vacant site (373 TfO salt) while the hydride remains intact. Complex 362 (TfO salt) also reacts with nitrosobcnzene to give 374 (TfO salt), which was investigated crystallographically and by electron spin resonance (ESR) techniques. ... 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

As expected, many unsaturated transition metal hydride complexes catalyse isomerisation. Examples include monohydrides of Rh(I), Pd(II), Ni(II), Pt(II), Ti(IV), and Zr(IV). The general scheme for alkene isomerisation is very simple for instance it may read as follows (Figure 5.1) ... 

Scheme 5 shows a formal explanation of this conceptual idea for alkanes developed by Wilhams [60] the alcohol is first oxidized into the aldehyde affording a hydride complex. The aldehyde is then condensed with the Wittig reagent to form the unsaturated compound, which becomes hydrogenated by the hydride complex, thus regenerating the catalyst. 

Hydride complexes of the transition metals occupy a central role in contemporary chemistry, both because of their importance as catalytic or stoichiometric reagents for fundamental organic transformations (e g., the catalytic hydrogenation of unsaturated systems) and because of their chemical interest per se. 

Metal hydride formation from the hydroxycarbonyl can be a concerted process, involving -elimination of metal and hydrogen, leading directly to the products. A requirement of this pathway however, is that the metal hydroxycarbonyl species, (18), be coordinatively unsaturated. A complex lacking this prerequisite cannot form a metal hydride directly. 

The distinction between coordination and organometallic chemistry, implicit in the title of this series, is not clear cut for hydride complexes because so many coordination complexes of the hydride ligand either react with unsaturated organic compounds to give, or are formed from, organometallic species. We shall therefore cover all aspects of hydrides, while emphasizing coordination chemistry. 

The chief avason for utilizing the reaction sequence described is the fact that formation of the a,p-unsaturated ketone in II makes it possible to dilTea ntiate between the two double bonds, because the copper hydride complex [PPh CuH], reduces only the conjugated double bond... 

The main synthetic methods used in the preparation of the coordination compounds mentioned above are reactions of coordinatively unsaturated complexes with hydrogen, protonation of hydride complexes, and formation of complexes containing molecular hydrogen by a synthesis reaction under reducing conditions. 

---