The Structures and Bonding of Hyperlithiated Molecules

This article describes the nature of bonding in hyperlithiated molecules with stoichiometries exceeding normal valence expectations. The existence of such hyperlithiated or hypervalent molecules as CLi6, Li3O, Li4O, Li5O, Li3S, Li4S, Li4P, Li2CN, Na2CN, and K2CN has already been experimentally confirmed by means of Knudsen-effusion mass spectrometry in our laboratory. These molecules have nine or more valence electrons, violating, at least formally, the octet rule. However, these molecules are thermodynamically more stable than the corresponding octet molecules. Results of ab initio MO calculations reveal that the extra valence electrons beyond the usual octet are in singly occupied orbitals (SOMO) or highest occupied orbitals (HOMO) in the LinA (A = C, O, P, S) molecules. The SOMO as well as HOMO forms the Li–Li bonds between all pairs of lithium atoms and contribute to stabilization of these molecules. The bonding situation of the M2CN (M = Li, Na, K) species is apparently different from that of LinA. The favored structure has Cs symmetry and is best described as a complex of the CN− anion with the M2+ cation. The extra valence electron is in SOMO, which corresponds to the radical cation SOMO and contributions to M–M bonding. The presence of the M2+ unit is a typical example of justifying the term hypervalent.