Methyllithium and its Oligomers: Structural and Energetic Relationships.

Lithium compounds are known to associate in solvents, in the crystal and even in the gas phase. The detailed study of such compounds provides a convenient framework to understand the nature of the bonding in small clusters of these predominantly ionic species. In particular, methyllithium has aroused considerable interest as the simplest organolithium compound which also has a broad synthetic utility. Reliable structural and energetic data for methyllithium oligomers (CHsub 3 Li) sub n, n=1-4, have been obtained by ab initio calculations using relatively large basis sets and including correlation and zero point energy corrections. The dimer and the trimer have planar carbon-lithium frameworks whereas the tetramer has a capped tetrahedral geometrical arrangement. The association energies of the dimer, trimer, and tetramer are -44, -79, and -123 kcal/mol, respectively. The bonding in all these compounds is indicated to be predominantly (= 90%) ionic. Equilibria between the dimers, trimers, and tetramers have been modelled by semiempirical techniques using statistical thermodynamics and including solvent effects. The experimentally known negative temperature dependence of the equilibrium between the tetramer and the dimer in solution is due to entropic considerations involving solvated clusters. The mechanism of inversion of the methyl groups in methyllithium clusters is also discussed.