Disilenes pyramidalization

Thus, with an increase in the number of silicon atoms in a cyclic structure, analysis, in terms of the aromaticity concept, of the structural types and stability loses its prognostic value. A striking example of this is given by the pyramidalization of the Si atoms, typical of disilene, in hexasilaben-... 

From the reactions of carbon monoxide in frozen matrices with Me2Si155 and several arylsilylenes Mes(R)Si (R = Mes, 2,6-diisopropylphenoxy, f-Bu)156, adducts were formed (see Section V.A.2). Theoretical calculations led to consideration of both a linear silaketene and a pyramidal n-donor base complex structure for the Me2Si(CO) adduct249. The observation that warming the carbon monoxide adducts of the arylsilylenes led to the formation of disilenes was interpreted as indicating the formation of a nonplanar complex that could dissociate as do other silylene-n-donor base complexes156. 

The detailed analysis of the molecular structure of isomer A of trisilaallene 12 revealed the bent-allenic nature with two trans-bent Si==Si double bonds that share a silicon atom the Si -Si -Si bond angle is 136.49 (6)°, the two Si=Si bond lengths are in the range of those for typical stable disilenes, the two terminal silicon atoms are pyramidalized, and the dihedral angles Si -Si -Si -C and Si -Si -Si -C" are around 60°. 

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