Multiple Bonds to Silicon

Considerable literature is currently available about these species. The review by Raabe and Michl in 1985 on Multiple Bonding to Silicon 6 is certainly the most detailed and complete reference to the topic to that date. Subsequently a number of other reviews that provide additional details of more limited areas of the field have been written by Brook and Baines,7 Cowley and Norman,8 Gordon,9 Shklover et al.,w Grev,11 and Lickiss.12 In addition, much information is to be found in The Chemistry of Organic Silicon Compounds edited by Patai and Rappoport,13 especially in chapters 2, 3, 8, 15, and 17. 

G. Raabe, J. Michl, Multiple Bonds to Silicon, in S. Patai, Z. Rappoport (eds.), The Chemistry of Organic Silicon Compounds, John Wiley Sons, Chichester 1989, pp. 1015. 

The photolysis of various cyclic compounds containing silicon atoms in the ring have been found to lead to the formation of transient intermediates containing multiple bonds to silicon. For example, Boudjouk and coworkers observed that photolysis of the trisi-latriselenacyclohexane 330 gave rise to the silaselenone 331, captured as the insertion product into the ring of D3, yielding the product 332168 (equation 30). 

Experimentalists have been particularly well served by numerous reviews from some of the leading workers in the field. For example, Brook and Baines (76) have reviewed silenes, Wiberg (77) discussed M=C and M=N double bonds (M = Si and Ge), Cowley and Norman (78) have reviewed M=M (M, M = group 14 and 15 elements) double bonds, Raabe and Michl (79,80) have provided complementary reviews of multiple bonds to silicon, West (81) has reviewed disilene chemistry, Masamune (82) has reviewed the work of his group on Si=Si and Ge=Ge compounds, and most recently Barrau et al. (83) have summarized multiple bonds to germanium. Studies from the reactive intermediates era of this field are beautifully summarized by Gusel nikov and Nametkin (84). 

Silylenes and Multiple Bonds to Silicon Synergism between Theory and Experiment... 

The field of silicon chemistry has enjoyed a very fast development in the last two decades with many novel significant discoveries being made [1]. Of particular interest in the context of this paper is the synthesis and characterization of a variety of reactive intermediates such as silylenes [2] and compounds with multiple bonds to silicon [3]. These exciting developments were occurring at the time when theory, in particular ab initio molecular orbital theory, was reaching "maturity" i.e. at the time when these methods could be used routinely to calculate reliably the properties of a variety of molecules, including silicon compounds [4]. 

We have shown in this paper that molecular orbital calculations at the ab initio level can be used to predict reliably the spectral transitions in silylenes, to evaluate the effects of substituents on the Si=Si multiple bond, to shed new light on existing experimental data and to direct future work towards the synthesis of novel isomers of disilenes. Although carbon and silicon are isoelectronic, multiple bonds to silicon differ dramatically from multiple bonds to carbon and analogies from carbon chemistry might therefore be entirely misleading when applied to silicon compounds. We believe that our studies have demonstrated the enormous power of modem computational methods and hope that this paper will prompt future theoretical studies and more importantly, theoretical-experimental collaborations in the field of organosilicon chemistry. 

There has been considerable interest during the past decade in the study of compoimds with multiple bonds to silicon. In particular Si=Si and Si=C double bonds have been studied extensively both experimentally and theoretically [1]. In contrast, relatively little is known about triple bonds to silicon and a stable compound of this family has not been prepared yet. However, there are three reports on the spectroscopic identification of transient compounds with Si-N triple bonds, the silanitrile, HSiN [2], the silaisonitrile, HNSi [3], and its phenyl substituted derivative, PhNSi [4] (the latter formally possess an Si=N double bond). Calculations for the parent silanitrile/silaisonitrile system [3d, 5] and for their phenyl-and methyl-substituted derivatives [6] predict that RSi=N, which formally contains a Si=N triple bond, is significantly less stable than the isomeric silaisonitrile, RN=Si, and that the rearrangement barriers of RSiN to RNSi are relatively small e.g., for R = H the barrier is only 8.3 kcal mol" at G2 [3d]. This stability order is opposite to that in the isovalent carbon analogs HCN/HNC [7]. However, despite the... 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

At temperatures above 140 K, the cSiCs radical cations were converted into the neutral radicals via geoselective deprotonations. In recent years, a number of compounds which show multiple bonding to silicon has been isolated and studied [248]. Monomeric silanones, 89, have as yet been observed only in solid matrices [249], apparently the presence of two bulky groups is not sufficient to prevent oligomeri-... 

G. Raabe, J. Michel, Multiple bonds to silicon, in The Chemistry of Organic Silicon... 

No silicon compounds containing multiple bonds to silicon are known. The hydrolysis of halogen compounds of silicon (discussed later) does not yield the silicon analogues of ketones and carboxylic acids by elimination of water from each molecule. Instead, water is eliminated from two or more molecules leading to polymerized products. 

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