Tetramer, methyl lithium

The strong preference for oxygen over carbon as a donor atom is shown by the rapid decomposition in water of most compounds with a covalent M—C bond or an ionic M+. .. C- interaction. The effect of the differences in electronegativity is shown in the structures of methyl lithium and methyl potassium. The methyl lithium tetramer, (39), has a cubane ... 

Spherical-domain models of three-center bonds in localized-molecular-orbital models of a nonclassical carbonium ion, B4CI4, and TaeClfJ have been described 49,52) a drawing of a spherical-domain model of the methyl lithium tetramer, (LiCH, is shown in Fig. 31. Large, outer circles represent domains of electron-pairs of C—H bonds. Solid circles represent domains of Li+ ions. Shaded circles represent 4-center lithium-lithium-lithium-carbon bonds — i.e., electron-pair domains that touch, simultaneously, three lithium ions and the kernel of a carbon atom. The... 

Another tetramer is methyl lithium. MeLi is a very reactive compound in the monomeric state, and it crystallizes as a tetramer a tetrahedron of lithium atoms with a methyl group plugged in to the centre of each face. 

The structure of the methyl lithium tetramer. Only two methyl groups have been... 

Structurally similar tetramers are maintained in solutions with electronic donors (THF or TMEDA). For example, the same core structure exists for the methyllithium TMEDA adduct. Ab initio methods have been used to study the aggregation of various methyl lithium oligomers in the presence of various donor ligands. The heavier alkali metal (M = K, Rb, Cs) methyl compounds have more ionic... 

Li The only shift obtained by double resonance is that in the methyl-lithium tetramer. (371)... 

Fig. 21. Tetrahedral structure of the methyl-lithium tetramer. The gray balls represent the methyl group. Each methyl group interacts with three Li atoms...

The Sign of the Lithium-Carbon Nuclear Spin Coupling Constant in Methyl-lithium Tetramer, W. McFarlane and D. S. Rycroft, J. Organometal. Chem., 64, 303 (1974). 

An alternative interpretation of these fractional kinetic orders in alkyl-lithium concentration as proposed by Wakefield [19] is that the rate-determining step involves coordination of a DPE molecule to one face of the polyhedral organolithium aggregate. As suggested previously [8], incomplete or stepwise dissociation equilibria such as those shown in Scheme 3 would be expected to require less energy as predicted by theoretical calculations [32]. It is important to note that Brown and coworkers [34, 35] have reported that dissociation energies for tetramer-dimer equilibria are 46.1 kj/mol and lOOkJ/mol for methyl-lithium in ether [34] and ferf-butyllithium in cyclopentane [35], respectively. 

It should, however, be pointed out that even if the alkali metal alkyls were all ionic, the bigger the cation and the smaller the anion the smaller would be the solubility. For example, the methyls of lithium and sodium form tetrameric aggregates (MeM)4, the structure of the lithium compound being shown in Figure 9. The insolubility of methyl-lithium in hydrocarbons could well be due largely to electrostatic interaction between the positive lithium atoms of one tetramer and the negative carbon atoms of a neighbouring tetramer. Tertiary butyl-lithium also... 

Figure 9. Unit cell of Methyl-lithium. The (CH3Li>4 tetramer. (After E. Weiss and E. A. C. Lucken, /. Organometal Chem, 2, 1964, 197.)...

Ethyl-lithium has a more complicated structure, the lower symmetry of the ethyl groups resulting in linear (instead of three-dimensional) association of tetramer units. t-Butyl-lithium is tetrameric in solution, and a tetrahedral structure like that of methyl-lithium is indicated by its infrared and Raman spectra. The bulk of the t-butyl group greatly reduces interaction between tetramers, and (Bu Li)4 sublimes at 7070 1 nun and is much more volatile than (MeLi)4. 

Fig. 7.3. Crystal structures of some lithium etiolates of ketones. (A) Unsolvated hexameric enolate of methyl t-butyl ketone (B) tetrahydrofuran solvate of tetramer of enolate of methyl r-butyl ketone (C) tetrahydrofuran solvate of tetramer of enolate of cyclopentanone (D) dimeric enolate of 3,3-dimethyl-4-(r-butyldimethylsiloxy)-2-pentanone. (Structural diagrams are reproduced from Refs. 66-69.) by permission of the American Chemical Society and Verlag Helvetica Chimica Acta AG.

Figure 4.17 Crystal and molecular structure of (LiMe)4 showing (a) the unit cell of lithium methyl, (b) the LijCj skeleton of the tetramer viewed approximately along one of the threefold axes, (c) the 7-coordinate environment of each C atom, and (d) the (4 -I- 3 -I- 3)-coordinate environment of each Li atom. After ref. 93, modified to include Li—H contacts.

A great deal of effort has been directed to determining the structures of lithium alkyls. It has been determined that in hydrocarbon solutions the dominant species is a hexamer when the alkyl groups are small. In the solid phase, the structure is body-centered cubic with the (LiCH3)4 units at each lattice site. Each unit is a tetramer in which the four lithium atoms reside at the comers of a tetrahedron and the methyl groups are located above the centers of the triangular faces. The carbon atoms of... 

Dimethylamino)methyl]-5-methylphenyl]lithium is a white crystalline solid and is pyrophoric in air. Furthermore, it is soluble in hydrocarbons and ethers. Molecular weight determinations in benzene have established a tetrameric structure.12 In THF, like [2-[(dimethylamino)-methyl]phenyl]lithium, this tetramer breaks down to a dimeric species. 

Furthermore we have quantitatively analyzed the C—Li bonding mechanism in the CH3Li monomer 25, dimer 26 (not discussed here), and tetramer 27 (see later) through a decomposition of the overall bond energy AE. The latter corresponds to the formation of (CH3Li) from the corresponding methyl and lithium radicals and is made up of two major components (Eq. [34]). 

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