Are Polar Organometallic Compounds “Carbanions”? The Gegenion Effect on Structure and Energies of Alkali-Metal Compounds

Polar organometallic compounds such as alkyllithium compounds or Grignard reagents often are conceived as “carbanions”, although it is well known from experiments that the metal gegenions may have a strong, and often directing, influence on the reactivity (for example, the basicity vs. the nucleophilicity) of the “carbanion”. This demonstrates that “carbanions” are tightly associated ion pairs, at least in commonly used less polar or nonpolar solvent systems like ethers or hydrocarbons. However, despite about ninety years of widespread application of these organometallic compounds as bases or nucleophilic reagents, the role of the gegenion is not yet understood fully. The focus of this review is on the inherent influences of the metal gegenion on the structures, reaction energies, and activation barriers of some representative alkali-metal compounds. While most comparisons of physical and chemical properties determined experimentally for Group I compounds are precluded due to different substituents, solvents, ligands, etc., ab initio calculations of model compounds can yield intrinsic data that are useful for a better understanding of chemical behavior. For this reason, most of the data reviewed in this article are based on quantum chemical calculations, which are compared with experimental data where available.