Fig. 1-5 The extent of the electron orbit of the hyperlithiated molecule Li4S. The large black circle denotes a sulfur atom, the small black circles denote lithium atoms, and it is seen that the electron orbit contributing to the Li-Li bond envelopes the whole molecule.

Usually a carbon atom establishes 4 chemical bonds as in CH4. This is explained by the octet rule that molecules composed of the second period elements of the periodic table such as C, N, and O are most stable when 8 electrons exist in the valence orbit. We found experimentally, however, that when the hydrogens in a hydrocarbon are replaced by lithium (with similar chemical characteristics to hydrogen), the number of electrons in the valence orbit can sometimes be less or more than 8 as in CLi6. The existence of this kind of exotic molecule is also confirmed theoretically by high-accuracy, quantum mechanical calculations, and the possibility of the existence of many other hyperlithiated molecules with 9, 10, or 11 valence electrons are also theoretically predicted. For example, in comparison with the octet molecule Li2S the Li-Li bond becomes stronger than the Li-S bond in the case of the hyperlithiated molecules Li3S or Li4S and the electron orbit contributing to this bond envelops the whole molecule, which is easily understood from the computer graphics (Fig. 1-5). It is expected that the clarification of the bonding state of these molecules will contribute to the development of the theory of chemical bonding.

Reference H. Kudo, Observation of Hypervalent CLi6 by Knudsen-effusion Mass Spectrometry, Nature, 355, 432 (1992).

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