29Si and 13C-NMR chemical shifts in crowded bis (trimethylsilyl)ethenes

29Si and 13C-NMR chemical shifts are presented for the three isomers of bis(trimethylsilyl)ethene and they are compared with the shifts in vinyltrimethylsilane and in the carbon analogues. Methyl carbon chemical shifts in trimethylsilyl substituted ethenes parallel the trends in the shifts in the carbon analogues and together with the shift of the quaternary carbon in the tert-butyl substituted ethenes they exhibit paramagnetic steric shifts similar to those observed in other compounds containing such substituents in similar sterical arrangement. With the exception of cis-1,2-bis(trimethylsilyl)ethene, the silicon chemical shifts vary in trimethylsilyl substituted ethenes as the quaternary carbon shifts in tert-butyl substituted ethenes. In cis-1,2-bis(trimethylsilyl)ethene silicon shows diamagnetic steric shift but the silicon in 1,1-bis(trimethylsilyl)ethane is deshielded relative to vinylsilane. Using empirical correlations of olefinic carbon shifts it is demonstrated that both steric and electronic effects are important in bis(trimethylsilyl)ethenes and a consistent interpretation of the observed trends is offered. (author)