Tetramerization of CH3Li
From WikidChem
- This discussion needs work to identify the two high OMOs and two low UMOs in each of two (CH3Li)2 dimers and how the two dimers orient so that the UMOs of one stabilize the HOMOs of the other, and vice versa. This still leave 4 low UMOs in the tetramer that can be used to stabilize unshared electron pairs on ether oxygens. As it stands none of this is expressed clearly. - JMM
CH4Li can aggregate into tetramers because a cubic formation allows for the best overlap between the pi Li-C and sigma C-H. Li has a high energy because of its low nuclear charge, so it forms a high HOMO with Carbon. In this molecule, the sigma C-H antibond is the low LUMO.
Li has 3 empty P orbitals. A lobe of one of these orbitals is occupied by electrons from the C-Li bond. The remaining two orbitals are empty, and are orthogonal to the other two P orbitals. Maximum P orbital overlap occurs when the P orbitals are parallel to each other, so the two unoccupied P orbitals will align perpendicularly to P orbitals on other CH4Li molecules. These orbital mixings form a 90 degree right angles that are the vertices of the cube.
Only one lobe of one of Li's P orbitals was employed in bonding with Carbon, so the other lobe is still unoccupied and unreactive. When exposed to an excess of ether (CH3OCH3), which has lone electron pairs on the oxygen ketone, these empty orbitals will be filled with electrons from this oxygen. If there's enough ether, the cubic tetramer will want to break apart because sharing electrons with the ether's oxygen is more thermodynamically favorable.
- Note: sigma and pi are used in these slides to denote the symmetry with respect to planes containing the C-Li axis, or the principal plane of the dimer. Pi orbitals change sign upon reflection across such a plane. As MOs they may be bonding, anti-bonding, or non-bonding (neither favorable nor unfavorable overlap between atoms). - JMM
