Electron-Deficient Bonding beyond B2H6:

Calculated Structure of Methyl Lithium Tetramer
with a molecule of Dimethyl Ether

Because the lithium atom in CH3Li has three vacant orbitals, it is on the prowl to stabilize up to three high energy electron pairs in addition to those of the previously formed C-Li bonding electrons. This results in the following complex tetramer with an appended solvent molecule. This precise structure comes from a quantum-mechanical calculation (see footnote) but can also be observed experimentally.


Note that this structure can be understood as a tetrahedron of lithium atoms interlaced with a tetrahedron of methyl carbons to form, overall, a distorted cube.

The coloring in the following version of the picture shows how red and maroon (CH3Li)2 dimer "squares" (analogous to the B2H6 dimer of BH3) came together along the blue bonds to form the "cube". Each blue (or red or maroon) bond can be thought of as a lithium vacant orbital going after a carbon HOMO to stabilize it. (In the square dimer the carbon HOMO is already stabilized by red and maroon Li orbitals.)

The oxygen of dimethyl ether forms a bond with one of the lithium atoms. Presumably if there were four ethers in the calculation, each lithium would be so decorated.

Although there seem to be 12 C-Li bonds within the distorted cube, there are only 4 electron pairs to accomplish this bonding (hence "electron-deficient" bonding). Each electron pair accounts for 3 of these bonds. Two of the four 4-branched version of the "Y" bond are shown by the blue and red lines added to the doctored figure below. Each of these can be thought of as the sharing of the "unshared pair" of a CH3 anion with three Li cations. Each lithium is involved in three such 4-branched bonds, while the fourth vacant valence-shell hybrid orbital of one lithium is used to stabilize an unshared pair of the oxygen in the ether at the right.

original figure taken from: http://www2.chemie.uni-erlangen.de/external/cic/tagungen/workshop95/hommes/, no longer supported