Bonded Silicon Compounds

The most interesting general conclusions reached from these studies are the following  

TABLE 14. Bond lengths and distance shortening on going from single to double bonds0  

Bond Single bond Double bond Shortening 

6 At 3-21G, all values in A. Similar values are obtained at 6-31G, see Reference 175b. 

The small energy differences between double-bonded and divalent species is a general important feature of silicon compounds. It indicates that silicon is reluctant to form doubly bonded compounds and this is responsible for many of the differences between carbon and silicon chemistry. It can be generalized that for n = 0-2, H2C=XHn is much more stable than HCXH +l, while H2Si=XH and HSiXHn+l have similar stabilities. 

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Polyolefins are, of course, usually synthesized by the catalyzed polymerization of alkenes. Why is an analogous route, polymerization of disilenes, not employed to prepare polysilanes The reason is paradoxical. The energy barrier to polymerization of doubly bonded silicon compounds is simply too low, so that in most cases they polymerize (or oligomerize) immediately when they are generated ... 

B. T. Luke, J. A. Pople, M.-B. Krogh-Jespersen, Y. Apeloig, J. Chandrasekhar, and P. v. R. Schleyer,/. Am. Chem. Soc., 108,260 (1986). A Theoretical Survey of Singly Bonded Silicon Compounds. Comparison of the Structures and Bond Energies of Silyl and Methyl Derivatives. 

Stable triply-bonded compounds with group 14 elements are still unknown. Several formally triply-bonded silicon compounds were detected and characterized in matrix and... 

Second, chapters on topics which were covered incompletely or partially in Vol. 2 were extended here by including new sub-topics related to the same themes. These include 29Si NMR, ion-molecule reactions of silicon ions and the reactivity of multiply-bonded silicon compounds. 

Physical Properties of Selected Stable it-Bonded Silicon Compounds ... 

Reviews on multiple bonded silicon compounds Si=Y see Adv. Organomet. Chem. 1995, 39, (Eds. R. West, F. A. Stone). 

FIGURE 15. Schematic description of the energy profiles for addition reactions to doubly bonded silicon compounds. Reproduced by permission of Kluwer Academic Publishers from Ref. 175b. 

Both experimentally and theoretically triply bonded silicon compounds proved to be a harder nut to crack than doubly bonded silicon compounds. Strong experimental evidence for the existence of triply bonded silicon comppunds such as RSi=CR or RSi=SiR is not yet available (however, HN=Si has been identified, see below), although some indications that they may exist as transient species have been presented19. In this situation, theory remains the only reliable source of information for these molecules that allow one to study their fundamental properties and reactions. Furthermore, the calculations can be used to develop and test ideas, which may eventually lead to the synthesis of stabilized molecules of this family that may have a better chance to be observed experimentally. As will be demonstrated in the discussion below, theory has faced many difficulties in studying these molecules and it seems that only in the last two years the calculations are converging towards a consensus about the structures and relative stabilities of these molecules. 

Preparative methods leading to multiply bonded silicon compounds and the reaction modes of such compounds have been summarized in a series of charts. In these charts, labels at the reaction arrows refer to the subsection or subsections in which the reactions are discussed. 

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