Silicon-carbon compounds silylenes

The 13C-NMR spectra of 4-7, 9-11 show a close similarity to the spectral data of analogous carbene complexes. The shift differences between the metal carbonyls of the silylene complexes and the related carbon compounds are only small. These results underline the close analogy between the silicon compounds 4-7, 9-11 and Fischer carbene complexes. This view is also supported by the IR spectral data. On the basis of an analysis of the force constants of the vco stretching vibration,... 

Divalent silicon compounds (silylenes), one of the most interesting class of low-coordinated silicon compounds, had been known as highly reactive, short-lived transient species that resembled the carbon analogues (carbenes), until the recent... 

No experimental evidence concerning compounds with silicon-carbon triple bonds is known, so that experimental efforts should be concentrated on the search for these fascinating species. 2-Silaallenes and 2-silaketenes, which also would contain a sp-hybridized silicon atom, could not be isolated up to now and remain a challenge of organosilicon chemistry (for 1-silaallene and for silylene-CO complexes cf Section VIII). 

The redistribution reactions mentioned above indicate that the opening of a coordination site not only promotes oxidative addition and possibly a-bond metathesis reactions, but — difG ndy from carbon compounds — also induces migrations of silicon substituents with the concomitant formation of silylene complex intermediates. It was pointed out by Tilley and co-workers that the transfer of a silyl substituent from silicon to platinum is easier in three- than in four-coordinated... 

For group 14, whereas divalent carbon and silicon (carbenes and silylenes) are typically highly reactive, divalent germanium is considerably more stable. Divalent tin salts are quite stable, although still relatively reducing. By contrast, divalent lead salts are very stable and tetravalent lead compounds are potent oxidants the cleavage of vicinal diols by lead tetraacetate is a good example of the latter ... 

The relative high stabilities of the pyramidal, bridged and silylenic structures in the case of SisHs result from several electronic effects typical of silicon, which were already discussed in detail in this review, such as the reluctance to form multiple bonds, high divalent state stabilization, preference for bridged bonding etc. These effects lead in the case of Si5H5+, as in many other examples discussed in this chapter, to stable three-dimensional structures which are not minima on the PES of the analogous carbon compounds. 

To a large extent the chemical shifts of carbon and silicon run parallel, but the chemistry of the two elements is somewhat different. Thus silicon can have extend its valence shell beyond the coordination number of 4. A few stable or-ganosilicon compounds in which silicon is divalent are known (the silylenes), and compounds with a silicon-silicon double bond also exist (the disilenes). 

Whereas the analogous carbenes easily isomerize wherever possible to compounds containing doubly bonded carbon atoms even under the conditions of matrix isolation, silylenes are almost as stable as the corresponding substances with doubly bonded silicon atoms. For example, methyl- and silylsilylene lie just 4 and 8 kcalmol-1 above silaethene and disilene, whereas the difference between ethene and methylcarbene is as high as 70 kcalmol-1 149-151 As a consequence, silylenes are often key intermediates on the way to other highly reactive silicon compounds discussed above. 

Compounds with two or more silicon atoms directly attached to one another, subdivided into sections based first on the number of silicon atoms and then on the carbon functionality attached to the silicon atoms. Frequently, but not exclusively, the main photochemical behavior involves homolysis of a silicon-silicon bond yielding silyl radicals, but in some cases silylenes result directly from the photochemistry. The resulting compounds are frequently the products of a molecular rearrangement. 

Many of the results reviewed here suggest that a replacement of the usual alkyl or aryl substituents by silyl substituents in unsaturated silicon and germanium compounds may be rewarding. As we noted, silyl substituents do tilt the properties of silylenes, silyl radicals, and sequential BDE trends toward those in carbon chemistry. They have already been shown to stabilize disilenes with respect to dissociation to two silylenes, and this may be crucial to the further development of digermene and distannene chemistry. 

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. 

Stable compounds containing simple three-coordinate silicon atoms other than the alkali metal salts were not isolated until a range of transition metal silylene complexes were prepared. However, instead of having simple structures of the type R2Si = ML , that is, analogous to their carbon analogs, the silicon atom is usually further coordinated by a donor ligand such as HMPA or THF. Some donor-free complexes have, however, been prepared in solution. 

The formal substitution of a saturated carbon atom in compounds 8, 13 and 16 by silicon results in precursors 21, 17 and 20, which could all be synthesized. In contrast to the results above, pulsed flash pyrolysis of these oligosilanes gave rise to the formation of only one C2H4Si2 isomer, namely 18. Actually, irradiation of matrix-isolated 2-silylsilacyclopropenylidene (18) led to silylene 19 in analogy to reaction 14—>15. 

Because of the increased separation of the valence s and p orbital energies, the stability of the divalent state increases down the group. Thus, whereas divalent carbon (carbenes) and silicon (silylenes) compounds are generally unstable, there are many relatively stable divalent compounds of Ge, Sn, and Pb compounds. Divalent Ge and Sn compounds, however, are strong reducing agents. By contrast, divalent Pb... 

SiUcon-carbon and silicon-nitrogen multiply bonded compounds 899 method for the synthesis of silylenes ". Silene 258 is not obtained (equation 64). 

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