Metal-silicon bonds

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

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. 

This method offers a new route to the formation of transition metal silicon bonds. 

Transition-metal chemistry is currently one of the most rapidly developing research areas. The record of investigation for compounds with metal silicon bonds is closely comparable to that for silicones it was in 1941 when Hein discovered the first metal silicon complex, followed by Wilkinson in 1956. A milestone in the development of this chemistry was Speier s discovery of the catalytic activity of nobel metal complexes in hydrosilylation reactions in 1977. Hydrosilylation is widely used in modem organic syntheses as well as in the preparation of organo functionalized silicones. Detailed investigations of the reaction mechanisms of various catalysts continue to be subject of intense research efforts. 

The preparative potential of silyl- or stannyl-substituted polynuclear complexes is currently far from being exploited. Due to the reactivity of silyl ligands, selective cleavage of metal-silicon bonds is possible. In some cases this was observed during the reaction of an anionic silyl complex with metal... 

New Chemistry for Reactive Transition-Metal Silicon Bonds... 

All alkali metal hypersilanides display highly pyramidal central silicon centers with Si-Si-Si angles around 100°. Remarkably, these angles do not differ in the contact or separated ion pairs, further indicating the large degree of charge transfer from the alkali metal cation to the silanide anion. The pyramidalization is also an expression of the increased />-character of the Si-Si bond, and consequent increase in the alkali metal-silicon bond. 

The introduction of a group IV metal into transition metal complexes modifies the behavior of these complexes and a wide variety of reaction sites may result. Thus, reactions at silicon without cleavage of the transition metal-silicon bond are possible in some cases. 

Most of the above reactions occur via a mechanism involving intermediates with a metal-silicon bond (i.e. silicometallics) and a metal-hydrogen bond, accompanied (or sided) only occasionally by compounds containing metal-carbon bonds (i.e. organometallics) that are characteristic of the key intermediates of transition-metal-catalyzed transformations of organic compounds (for recent reviews, see Refs [11, 13]). 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

For a recent analysis of the nature of the metal-silicon bond in neutral transition-metal complexes, see H. Jacobsen and T. Ziegler, Inorg. Chem., 35, 775 (1996). 

During the last five years, a vast amount of research has been focused on the chemistry of group-8 silicon complexes. This section takes into account only those complexes containing well-characterized metal-silicon bonds but does not take into account intermediary compounds in stoichiometric or catalytic reactions in which such species may be formed. 

The first mercury(I) silyl complex [(Me3SiMe2Si)3Si]2Hg2 was prepared by the reaction of an excess of (Me3SiMe2Si)3SiH with (t-Bu)2Hg. The solid structure of the complex displays linear Si—Hg—Hg—Si fragment with regular metal-silicon bond lengths... 

Metal-shielding tensors, for quadrupolar nuclei, 1, 473 Metal-silicon bonds... 

Two quantitative applications may be noted in the first (entries 12 and 14), infrared intensity measurements are related to the angle of tilt, 6, between equatorial CO groups and the metal-silicon bond [see formulas (XXXIV) and (XXXV)]. Thus in the series Me3MCo(CO)4 the... 

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