Silaallenes complexes

Reiterating the idea that the -complexes 132a and 86a are not truly heteroallenes, one must consider the central carbon C NMR chemical shifts. Silaallene complex 132a starts to approach allene status with a chemical shift of 175.5 ppm, but the most deshielded carbon for alkylidenetelluragermirane 86a is only 153.04 ppm. 

An example of particular interest is the two-fold introduction of M(CO)n moieties at silicon to give HMPA adducts of organometallic analogues of silaallene. It has been shown that this reaction proceeds through the dichlorosilylene complex as intermediate. Both the iron 22 and ruthenium 23 compound and also the bimetallic complex 24 are accessible. 

The Kumada/Ishikawa group also investigated thermolytic reactions of alkynyl-polysilanes and silacyclopropenes in the presence of nickel catalysts and implicated a 1-silaallene-nickel complex as an intermediate in the reaction pathway... 

Numerous other studies " by Ishikawa and co-workers, with or without nickel catalysts, have reinforced the importance of l-silaallenes and nickel-complexed 1-silaallenes as intermediates in the pathways of the photolyses and thermolyses of alkynylsilanes. 

One other study of group 14 heteroallenes involving transition metals was reported in 1995. Jones et al. described the isolation of a ruthenium complex of a 1-silaallene (132—Scheme 32). The 1-silaallene also interacts with a hydrogen atom as well as the ruthenium metal center. Jones et al. describe this view... 

The structures of two group 14 heteroallenes that are complexed to other atoms have been determined—a ruthenium complex of a 1-silaallene (132a) and a... 

If doubly-bonded compounds >M = X (M = Si, Ge, Sn) are now well known, this is not yet the case for the heavy allenic derivatives >M = C = X. Transient silaallenes >Si = C = C 160-167 or silaazaallene >Si = C = N—168 were postulated some years ago, but the first stable derivative of this type, a formal 3-stanna-l-azaallene >Sn = C = N —, was isolated only in 1992 by Griitzmacher et al169 The compound Tbt(Mes)Si = C = NR170 has also been obtained, but according to the authors it is closer to a silylene Lewis base complex than to a silacumulene. The first stable silaallene was only reported in 1993 by West et al.111 and the first metastable >Si = C = P— was characterized still more recently.172... 

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). 

Ishikawa and coworkers have investigated rather more complex systems in which mixtures of disilylalkynes have been co-photolyzed with trisilanes using a low-pressure mercury lamp. The main reaction was one in which the silaallene (from the disilyl-alkyne) combined with the silylene (from the trisilane) to give remarkably stable solid disilacyclopropanes53 (equation 31). 

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