Silyl cations stabilized by oxygen-donors

The first hydrazide-based siliconium-cation salts were obtained during an attempt to prepare neutral, hexacoordinate bis-chelates with O Si coordination (80), in analogy to the extensively studied isomeric N Si coordinated 30-38 (see Section III.A.l). In analogy to Eq. (15), the A-trimethylsilylhydrazides 81 were allowed to react with polyhalosilanes 22-25 (Eq. 33). However, in contrast to Eq. (15), the expected 80 were not 

The salts 82-85 were characterized by their typical pentacoordinate 29Si chemical shifts (Table XXV), by the equivalence of the two chelate rings resulting from the molecular C2 symmetry, and by spectral analogy with a binuclear disiliconium dichloride salt (86) for which a crystal structure (Fig. 46) was obtained. 

Compound 86 exists in two diastereomeric forms due to the chiral silicon centers, which are evident in the various NMR spectra (Table XXV). The crystal structure of 86 shows a molecular inversion center, and hence belongs to the meso diastereomer. The geometry around silicon in the crystal is a distorted TBP and the chloride is more than 7 A away from silicon, in accord with a pentacoordinate siliconium chloride structure. The dative Si bonds are exceptionally short in 86 (1.802 and 1.807 A) relative to those in other pentacoordinate complexes.2,3,1115 

The question as to why these O — Si coordinated chelates, 82-86, form donor-stabilized silyl cations, while the analogous N Si chelates (30-38) 

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