Coordination Modes for the Hydride Ligand

It is now becoming common to locate H atoms directly using X-ray diffraction methods, while neutron diffraction offers distinct advantages. Since H atoms scatter neutrons with about the same efficiency as do most elements, their positions are determined with high precision. Two distinct disadvantages of neutron diffraction are the necessity of a source of neutrons and of large single crystals. 

The first hydride complex investigated by the neutron diffraction technique was K2[ReH9] containing the terminal M—H linkage. Many examples of complexes containing the terminal hydride ligands are now known for virtually all d-block transition elements. Binary transition-metal hydrides are rather few and the majority are stabilized by carbonyl, phosphine, or other ancillary ligands. 

Generally, terminal-hydride complexes are prepared from metal halides, their phosphine derivatives, or organometallic complexes either by the action of a hydridometallate or dihydrogen in THF, alcohols, or other solvents. Potassium, sodium, sodium amalgam, and magnesium are also used as reducing agents. Some examples of preparative reactions are shown below 

Hydrolysis of complex salts and oxidative addition (p. 27) of weak acids are also utilized frequently  

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IR is seldom used to characterize silane complexes, although the stretching mode for the coordinated Si-H bond is observable by both IR and Raman. As for H2 complexes it is expected that this mode will be highly mixed with M-H types of modes and will be similar to vibrational modes for bridging hydride ligands. The position of the vM...H-si frequency can be as high as 1890 cm-1 but is generally in... 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

Using kinetic measurements, Konno et al. [100] have shown that the nucleophilic attack of the hydride ligand of Ru(tpy)(4,4 -X2bpy)HT (X = H, MeO) to the carbon atom of CO2 was the rate-determining step for the formation of the formate complex Ru(tpy)(4,4 -X2bpy)(OCHO) , whereas the coordination of CO2 to the metal, either in q -O or q -C,0 mode, does not play any important role in the transition state. 

Hz) are observed for a cis-phosphorus-hydride coordination mode. If there is fast exchange of the phosphorus ligands between the equatorial and apical positions, an averaged resonance and coupling constant are observed for the equilibrium mixture of the ee and ea isomers. 

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