Hypervalent and Related Species

Schleyer, Pople and coworkers have shown calculationally that polylithiated methanes with one or two additional lithium atoms should form very stable hypervalent species of high symmetry. Thus both trigonal-bipyramidal CLij 44 (Dj symmetry) and octahedral CLig 45 (O, symmetry) deduced from CLi 5 are indicated by ab initio calculations (3-21G basis set) to be highly stable toward loss of a lithium atom from 44 or loss of Li from 45  

The related series of molecules deduced from CHLij 4 and CHjLij 3, respa tively, behave similarly CHLi 4(5 (C, symmetry), CHLij 47(C, symmetry), CH Lij 45 (Cj symmetry), and CHjLi 49 symmetry). 

The charge on carbon, however, does not increase appreciably as more lithium atoms are added. Thus the extra electrons beyond the usual octet are not associated with carbon but contribute to Li—Li bonding building a metallic cage around the central atom. Hypermetalated molecules therefore are not hypervalent in the strict sense. Additional calculations on hyperlithiated methanes have been performed by Waite and Papadopoulos as well as Weinhold et al. 

The corresponding hyperlithiated methanonium ions CH Li prefer the same 

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