_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The first three periodic groups
Table 3.2. Aggregation of organolithiums, (RLi)n in solution
RLi Solvent
Paraffins or Benzene or Diethyl Tetracyclohexane
toluene ether hydrofuran
MeLi Insol Insol 4 4
EtLi 6 6 4 4
Bu"Li 4-6 6 4 4
su•u 4 4
Bu 1 Li 4 4 4 4
PhLi 2 2
Menthyl Li 2
-Not known to author.
In (MeLi) 4 TMEDA (TMEDA = Me 2 NCH 2 CH 2 NMe 2 or tetramethylethylenediamine,
1. 6-dimethyl-2, 5-diazahexane ), (MeLi) 4 units are bridged by TMEDA
units. Bridging phenyl groups are present in the cluster [PhLi·OEt 2 ] 4 • Even in
donor solvents such as tetrahydrofuran or diethylether, aggregation of organolithium
species persists. The degree of aggregation, however, can vary with the
solvent (Table 3.2). Steric factors are clearly important; in general the most bulky
species are least associated. Cyclohexyllithium, however, can be crystallized from
benzene as a solvate which has the structure shown in Fig. 3.4, which is based on
a Li 6 octahedral cluster.
The el o sest Li-Li separations in these structures lie in the range 2. 3 7-2.7 4 A.
The covalent radius of lithium is 1. 34 A although the ionic radius of Li+ is only
o.76A.
Oligomers are present in the vapours above alkyllithiums. Mass spectroscopic
studies show that ethyllithium vaporizes as a mixture oftetramer and hexamer.
The bulk of the tert-butyl group reduces the interaction between tetramers even
more, so that (Bu 1 Li) 4 sublimes at 70°/0.1 mm; it is much more volatile than
either methyl- or ethyllithium.
Lithium can interact with n-systems in alkenes, alkynes and arenes. This may
account for the ability of lithium alkyls to initia te the polymerization of dienes
(p. 48).
Lithium forms numerous bridged mixed alkyl and aryl derivatives with other
elements. The structures ofLiAlEt 4 and ofLiBMe 4 are ofinterest on account ofthe
contrasting nature ofthe bridge bonding. The former resembles Et 2 Mg. The latter
consists of planar sheets of Li atoms bridged by BMe 4 units. Neutron diffraction
shows that the bridging to lithium occurs through the hydrogen atoms of the
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