Electronic properties hydrides

The hydrogen abstraction from the Si-H moiety of silanes is fundamentally important for these reactions. Kinetic studies have been performed with many types of silicon hydrides and with a large variety of radicals and been reviewed periodically. The data can be interpreted in terms of the electronic properties of the silanes imparted by substituents for each attacking radical. In brevity, we compared in Figure 1 the rate constants of hydrogen abstraction from a variety of reducing systems by primary alkyl radicals at ca. 80°C. ... 

The highly enantioselective reduction of benzils was achieved by the use of the chiral Ru complex (S,S)-28 with an S/C of 1,000 in a formic acid-triethylamine mixture to give the R,R diol in >99% ee (Scheme 34) [108]. The sense of enan-tioselection was the same as that of the reduction of simple aromatic ketones, suggesting that the adjacent oxygen atom does not participate in the stereoregulation. Introduction of electron-accepting functions at the 4 and 4 positions increased the reaction rate, while the enantioselectivity was not affected by the electronic properties of the substituents. Use of 2-propanol as a hydride source caused both the rate and enantioselectivity to decrease. An unsymmetrical 1,2-... 

Similar octahedral facial silyl methyl hydride complexes of the type IrL3H(SiR3)Me have been shown to induce competitive C—H/Si—C reductive elimination depending on the electronic properties of the silyl ligand, thus affording a novel example of a metallation of silyl ligands or the metallation of the sp3C—H bond of the ethyl moiety when R = OEt. For the complex with R = Et, mixtures of different complexes are formed by the thermolysis with benzene (Scheme 32)198,199. 

Complexes with two adjacent (cis) a-organyl ligands, or (more readily) a o-organyl and a hydride ligand, may decompose to provide hydrocarbon with concomitant (formal) reduction of the metal centre by two units. Either mononuclear (Figure 4.21a) or bimolecular (Figure 4.21b) decomposition routes may operate, depending on the steric and electronic properties of the particular metal complex. This has been most... 

The crystal lattices of the saline hydrides consist of hydrogen anions and metal cations, but not exclusively e.g., in LiH, calculations and diffraction experiments suggest that electron transfer from Li to H is 0.8-1 e, implying a strong ionic bond with covalent character. Magnesium hydride occupies a special position. Although classified here as a saline hydride, its properties are intermediate between the ionic hydrides and covalent BeHj-... 

The intensive research activities recently focused on late transition metal complexes as polymerization catalysts are justified by decisive improvements in the development of new polymer materials and indicate even more their high potential for future applications. However, up to now they often only dimerize or oligomerize a-olefms due to competing hydride elimination and associative olefin exchange reactions [19]. A new concept of new diimine ligands and complexes bearing 2,6-diphenyl-modified aniline moieties (Fig. 2.5) is stiU focused on sterically demanding substituents in the 2,6-positions, but it aims further towards the facile modification of the steric and electronic properties of the active species. 

In 1970 van Vucht et al. [41], in the Netherlands, discovered practical hydrides such as LaNis, and in 1974 Reilly and Wiswall, in the United States, discovered Fe-Ti hydrides [42]. In LaNis and other rare earth compounds such as YNis, NdNis and others, the plateau pressures are affected by the A-atom (example La atom) substitution. This led to numerous studies around the world and development of practical hydride work really started. There have been several excellent reviews over the years, among them notably references [2, 26,43-68]. Other notable papers on intermetallics in general, are those of Anderson and Maeland [63], Lou et al. [65, 66, 69], Bowman et al. [70], Cerney et al. [71], Percheron-Guegan and co-workers [29, 72], and Latroche et al. [73]. More recently an internet database has been set by Sandrock and Thomas (lEA/DOE/Sandia National Laboratory) [23], Jai-Young Lee and co-workers [74, 75], Uchida et al. [76], Yvon and Fichner (structural properties) [77], Gupta and Schlapbach (electronic properties) [78], and Flanagan and Oates (thermodynamic properties) [79]. 

---