Si-H2 complex

The E" " values were calculated from thermodynamic data. There are a large number of different acids, especially of the polyacid type. The complex forms exist in equilibrium, which depends on the pH. There are several compounds with hydrogen, the general formula of which can be given as Si H2 +2-Solubility equilibrium ... 

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

A major difference in comparison to M-H2 complexes is that M-H-Si and M-H-X linkages are asymmetric, i.e., look like hydride-bridged... 

The silane substrate would then displace H2 to give back the starting silane complex for further alcoholysis, and this was determined to be the rate-limiting step. These are all known reactions, and this mechanism and rate-determining step were recently supported by theoretical calculations that showed the heterolysis to be a highly concerted process, i.e., transformation of the a-silane complex to the H2 complex could even take place in a single step, thus circumventing the transient hydride complex (124). It is noteworthy that the mechanism of this reaction involves two different a complexes M(r 2 -Si-H) and M(r 2 -H2). 

By analogy to the well characterized rj2-H2 complexes, methane and silane can coordinate to the metal in an /x2-l ashion via a C-H a bond and a Si-H a bond ... 

Sigma complexes of Si-H bonds were first reported by Graham in 1969 althongh the nonclassical nature of the bonding was not recognized until after the first reports on H2 complexes. The early history of this field has been reviewed by Graham and Schubert, and the literature through 2001 has been reviewed by Kubas. ... 

A very useful monograph on H2 complexes, a key resource in the field, also covers alkane and silane sigma complexes M(jj -H-XR3) (X = C, Si), both fully covered in a recent review. ... 

Detailed information about the evolution of Si-H complexes in H ion-implanted silicon were gained by the infrared vibrational studies of Weldon et al. (1997) and Chabal et al. (1999). Implanted H atoms form complexes of the form VxHy or IxHy, where V denotes a silicon vacancy and / denotes silicon interstitial. Also observed was the so-called H2 complex, a hydrogen molecule formation in which one H atom is located at the bond-centered site and the other at the antibond site, with a silicon lattice atom residing between the H2 bonds. Upon annealing of the H ion-implanted samples, the IR studies uncover a net loss of bound hydrogen... 

The ri2-H2 or f/2-Si-H complexes could be relevant in Eq. (9.60), although the isolable complex Os(SiEt3)Cl(H2XCOXP,Pr3)2 proposed as an intermediate contains f/2-H2, which is favored over the OsH(if2-HSiEt3) structure in theoretical studies also (see Chapter 4). A related cationic complex [OsH(H2XCOXP Pr 3)2]+ acts as a template for a noncatalytic C-C coupling reaction between methyl pro-piolate and l,l-diphenyl-2-propyn-l-ol.116... 

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