Doubly coordinated silicon compounds

Disilenes exhibit the relatively low-field (8 = 49-155) 29Si chemical shifts characteristic of low-coordinate silicon compounds (Table I) thus 29Si NMR spectra are very important in their characterization. This deshielding is similar to that observed in the 13C chemical shifts of doubly bonded carbons relative to those of their saturated counterparts. 

In contrast to the extensively studied doubly bonded systems of silicon and germanium, the chemistry of the corresponding tin compounds, i.e. distannenes and stannenes, has not been fully disclosed yet probably due to the much higher reactivity and instability of such low-coordinated organotin compounds. In the following sections we briefly describe the synthesis and properties of stable distannenes and stannenes. 

Corriu and coworkers found that the reaction of the pentacoordinated functionalized silane 6 (see also Sections II.B.l and II.C.l) with elemental selenium leads to a stable silaneselone 140, which is stabilized by intramolecular coordination of the nitrogen-containing substituent to the doubly-bonded silicon (Scheme 42) . Although the structure of 140 was supported by Si and NMR and MS spectra, including a downfield Si chemical shift (S = -1-29.4) and a high coupling constant with Se (JseSi = 257 Hz), neither the crystallographical structure analysis nor the reactivity of this isolable Si=Se compound has been reported. 

Table 19 presents Si chemical shifts and Si—H coupling constants for some of the cationic pentacoordinate silicon complexes and for some of their precursors. No clear trend can be seen for the Si chemical shifts of these compounds some are shifted downfield and others upheld, relative to their pentacoordinated neutral precursors. However, a trend seems to emerge from the one-bond Si—H coupling constants in the cationic pentacoordinate complexes V(Si—H) is generally greater than for neutral pentacoordinate complexes. This is possibly the result of the greater s-character in the Si—H bonds, in the doubly coordinated (sp -hybridized) siUcenium cations . 

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