LITHIUM SYNTHETIC REAGENTS: DIMERIZATION AND INTRAMOLECULAR ASSOCIATION. DOUBLE BRIDGING IN "DIANIONS"

Abstract- The dimerization energies of lithium compounds, 2LiX-(LiX)9, are quite large and depend principally on the electronegativity of X. Intramoleular association, the equivalent of dimerization, is comparably favorable energetically. Thus, many polylithium compounds, including many synthetically useful "dianions", are found by molecular orbital calculations to prefer structures in which the lithium atoms bridge symmetrically. An example is o,o'-dilithiobiphenyl, whose predicted structure has now been confirmed by X-ray spectroscopy. Dilithiated species based on propene (CH9CHCHLi9), propane (LiCH9CH9CH9Li), acetaldehyde (Li CHCHO), acetone (LiCH2COH Li ahd Li CHCOCH), ahd mies (e.g., HCOLi9) are amog 1,3-double bridging examles. Besies o,o-dilithiobiphenyl, 1,4-double lithium bridging is exhibited by 1,4-dilithiobutane, 1,4-dilithio-2-cis-butene, and 1,4-dilithio-1,3-cis,cis-butadiene. While electrostatic interactions provide the simplest explanation for these structures, multi center covalent bonding also is important in determining geometrical details and energetic differences