Heavy Analogs of Carbanions: Si‐, Ge‐, Sn‐ and Pb‐Centered Anions

Abstract

Standard organic chemistry textbooks typically define a carbanion as a highly reactive species with a trivalent carbon that has a lone electron pair and bears a negative charge. In contrast to electron-deficient carbenium ions (see Chapter 1), carbanions, featuring eight valence electrons around the central carbon, are electron rich being strong nucleophiles and Lewis bases, typically more powerful than amines. The best-known examples of carbanionic species are definitely the Grignard reagents RMgX and organolithium reagents RLi, readily accessible and extremely useful organometallic derivatives extensively applied in synthetic organic chemistry. Known for more than a hundred years, metal salts of carbanions have been thoroughly studied from the viewpoint of their fundamental properties, such as basicity, stability, ion-pairing behavior and aggregation states.Carbanions contain an sp 3 -hybridized anionic carbon adopting a trigonal-pyramidal geometry with one of its tetrahedral valencies occupied by a lone electron pair. Such geometry is highly reminiscent of that of amines, for example NH 3 , which is isoelectronic with the simplest methyl anion CH 3 − . This was indeed confirmed by PES measurements suggesting a barrier to inversion and pyramidal structure for CH 3 − , 1 that agreed well with the computational data, revealing an H-C-H bond angle for the methyl anion of 109.4 • . 2 The preference for the pyramidal geometry of carbanions can be readily understood from simple orbital considerations. Thus, in a planar carbanion the lone pair would occupy a pure p-orbital, whereas in a pyramidalized carbanion the lone pair orbital has a substantial s-character (close to sp 3 ) gaining an additional stabilization through the