Silicon metal-carbon bonds

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]). 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

In catalytic conversions of (organo)silicon compounds only the hydrosilylation is a well-known process of industrial importance [1-5]. However, since 1985 other reactions of silicon compounds catalyzed by transition metal complexes have been revealed and spectacularly developed. Most of them occur via a mechanism involving metal-silicon and metal-hydrogen bonds, only occasionally accompanied by (or sided) metal-carbon bonds. 

Silatungstacyclopropane (35) is subject to addition reactions as well. Methanol, H2, and H-TMS all add to the metal-carbon bond. The first two reactions lead to metal hydrides while the last reagent gives a metallasilane. Trimethylphosphine addition to (35) causes attack of cyclopentadienide at silicon and cleaves the silicon-metal bond <90JA6405>. 

The application of classical and non-classical (photoacoustic) reaction-solution calorimetry to probe the energetics of metal-ligand bonds in organometallic systems is briefly analysed and illustrated by thermochemical results involving two families of compounds. The classical reaction-solution studies enabled the discussion of the systematics of metal-carbon bond enthalpies in several complexes M(t) -C H ) L. The photoacoustic studies addressed the effect of phenyl goups on the energetics of silicon-hydrogen bonds in phenyl-substituted silanes. 

What type of compounds will be covered in this overview. Strictly, only compounds which have at least one metal-carbon bond (with the exception of metal carbides) are called organometaiiic. However, this definition was not adopted in this chapter because many families of compounds that are relevant in organometaiiic chemistry would not be included (e.g., alkoxides). By metal we mean any element from groups 1 (except hydrogen), 2, 3 (including lanthanides and actinides), 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 (except boron), 14 (except carbon and silicon), and 15 (antimony and bismuth only). Most of the available thermochemical data for all these species are freely available in a single on-line database the NIST Chemistry WebBook Unless stated otherwise, all the data included in this chapter were quoted from that reference. 

Organometallic compounds are those in which there is a metal-carbon bond. According to this definition, in the case of transition metals, this group of compounds includes not only metal carbonyls, olefin complexes, cyclopentadienyl, and other 7r-complexes, but also cyanide and fulminate compounds. Certain difficulties arise in defining the metal of the main group elements. Usually, organometallic compounds are comprised not only of compounds of typical metals, but also of metalloids such as boron, silicon, phosphorus, arsenic, selenium, etc. In compounds of metals as well as in those of metalloids, the bond is generally polarized as follows C. Consequently, the... 

In this book we are concerned with the properties of compounds which contain metal—carbon bonds. Traditionally organometallic chemistry includes the carbon compounds of the metalloids boron, silicon and arsenic, but excludes those of phosphorus and of other more electronegative elements. Metal carbonyls are discussed, but not cyanides or carbides, which are more usefully considered in conjunction with inorganic rather than organometallic compounds. 

---