Conjugation, effect electropositive

These studies show that the traditional approach for stabilizing silicenium ions by conjugating substituents (such as aryl groups2588) is not very effective. On the other hand, stabilization by electropositive substituents such as Li and BeH is very effective310. This is nicely exemplified by the fact that H2SiLiF is more stable as a silicenium ion-pair H2SiLi+F, where the positive center is stabilized by lithium, than in the covalent tetrahedral structure300. 

Ketenes can be stabilized both by conjugating and by bulky substituents, and each of these factors contributes to the stability of ketene. A computational survey in 1991 provided the insight that ketenes are stabilized by electropositive substituents, and destabilized by electronegative ones, and that there is an inverse correlation of ketene stabilization with substituent electronegativity. Substituents exert major effects on ketene stabilities and properties, for example, fluoroketene (FCH=C=0) is calculated to be the least stabilized monosubstituted ketene and exemplifies the destabilizing effects of electronegative and jr-donor substituents, but this ketene can nevertheless be generated and trapped in solution by reaction with imines (Scheme 7.38). 

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