Nearly planar nonsolvated monomeric silyl- and germyllithiums as a result of an intramolecular CH-Li agostic interaction.

Pale-yellow crystals of the nonsolvated monomeric silyl- and germyllithiums, tris[di-tert-butyl(methyl)silyl]silyllithium 2a and tris[di-tert-butyl(methyl)silyl]germyllithium 2b, were obtained by one-electron reduction of the corresponding silyl and germyl radicals with lithium in hexane. The crystal structure analysis of both 2a and 2b showed almost planar geometry around the anionic centers, due to both intramolecular CH-Li agostic interactions and steric reasons. However, the free anions [(tBu2MeSi)3Si-][Li+(THF)4] 3a and [(tBu2MeSi)3Ge-][Li+(THF)n] (n = 3, 4) 3b no longer showed a planar geometry, because of the absence of the intramolecular CH-Li agostic interaction. A temperature-dependent 1H NMR study of 2a showed that the CH-Li interaction is weak.