Reliable structural and energetic data for methyllithium oligomers (CH3Li),, n = 1-4, have been obtained by ab initio calculations using relatively large basis sets and including correlation and zero point energy corrections. The association energies for the dimer, trimer, and tetramer are -44.3, -79.0, and -122.9 kcal/mol, respectively. Monomeric methyllithium in the gas phase is indicated to be 88% ionic by natural population analysis and can well be described as a lithium cation stabilized methyl anion. The estimated heat of formation, MfoO = 26.9 f 0.5 kcal/mol, corresponds to a bond dissociation energy, Do(H3C-Li) = 46.4 f 2.5 kcal/mol. The bonding in methyllithium oligomers is largely electrostatic. Li-H ("agostic") interactions are responsible for the eclipsed conformation of tetrameric methyllithium in the gas phase. The staggered conformation found experimentally in the solid state is due to a packing effect involving interaggregate interactions. Rotation barriers of alkyl groups in (RLi), are about 1 (R = CH3) and 2 (R = C,H,) kcal/mol. Equilibria between dimers, trimers, and tetramers have been modeled by the semiempirical MNDO method using statistical thermodynamics and including solvation effects. Whereas trimers are favored over dimers in the gas phase, the opposite is true in solution. If tetramers dissociate to smaller aggregates in donor solvents, only dimers are expected. The experimentally known negative temperature dependence of the equilibrium (RLi)4 2(RLi), in solution is confirmed to be due to entropy. The inversion of alkyl groups in alkyllithium clusters does not involve dissociation or free carbanions but proceeds via polycoordinate carbon species with the critical substituents in a plane. The activation energy for this process in the tetramer is estimated to be 14.3 kcal/mol.

The octet rule predicts the stoichiometries of first-row hydrides remarkably well. Thus, "CHS" and "CH6" are only likely to exist as weak complexes between methane and a hydrogen atom or a hydrogen molecule. Surprisingly, when lithium is involved instead of hydrogen, the situation is completely different. Both trigonal-bipyramidal CLi5 (1, D3h) and octahedral CLi6 (2, Oh) are indicated by ab initio calculations using the 3-21G basis set and full geometry optimization2 to be highly stable toward all possible

Although the reactions of alkyllithiums with carbonyl compounds are among the most elementary synthetic reactions,1 23little detailed mechanistic information is available. By use of rapid injection NMR, McGarrity et al. have shown that butyllithium dimer in tetrahydrofuran reacts about 10 times faster than the tetramer with benzaldehyde.2,3 Even at high dilution, there is no detectable concentration of monomer.Hence, contrary to earlier suggestions based on kinetic evidence,4 the monomer does not appear to be the reactive intermediate in ether solvents.We now report an ab initio examination5 6of the mechanisms of model reactions of formaldehyde with the monomers CH3Li and LiH, as well as with their dimers. The performance of the split valence 3-21G basis set, used for all these systems, was evaluated at higher levels for the HLi + H2CO process (see caption, Scheme I) and should be reasonably reliable for the larger systems (Schemes II-IV).All four reaction pathways (Schemes I-IV) are rather similar; each proceeds in three stages. Formaldehyde complexes with the lithium reagents la-4a are formed first.4,6 Since the association Scheme I. Formaldehyde-HLi Addition Pathway0 1b 0 Energies in kcal/mol, distances in A, angles in deg. Data at 3-21G//3-21G are compared with optimized (6-31G*//6-31G*) as well as with single point [6-31 + G*//6-31G*] and {MP2/6-31 +G*//6-31G*} values.

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