Reactions of Silanes with Transition Metals

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. 

An important example in this context is the hydrosilation reaction [121]. Hydrosilation is the formal addition of a silane to an alkene in presence of a hydrosilation catalyst. This reaction has numerous applications e.g., with a suitable catalyst, an enantioselective hydrosilation is possible [122] and also hydrosilation and double hydrosilation of alkines [123] are known. 

A great number of articles related to the mechanism of this reaction has been published. It can be considered as certain that the silanes react with the platinum center by an oxidative addition to the metal with formation of a silylplatinum hydride and subsequent transfer of the silyl group to the coordinated alkene. 

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

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