Bonding in methylalkalimetals (CH(3)M)(n) (M = Li, Na, K; n = 1, 4). Agreement and divergences between AIM and ELF analyses.

The chemical bonding in methylalkalimetals (CH(3)M)(n)() (M = Li-K; n = 1, 4) has been investigated by making use of topological analyses grounded in the theory of atoms in molecules (AIM) and in the electron localization function (ELF). Both analyses describe the C-M bond as an ionic interaction. However, while AIM diagnoses a decrease of ionicity with tetramerization, ELF considers tetramers more ionic. Divergences emerge also when dealing with the bonding topology given by each technique. For the methylalkalimetal tetramers, the ELF analysis shows that each methyl carbon atom interacts through a bond pair with each of the three hydrogen atoms belonging to the same methyl group and through an ionic bond with the triangular face of the tetrahedral metal cluster in front of which the methyl group is located. On the other hand, the AIM topological description escapes from the traditional bonding schemes, presenting hypervalent carbon and alkalimetal atoms. Our results illustrate that fundamental concepts, such as that of the chemical bond, have a different, even colliding meaning in AIM and ELF theories.