Carbon bonding is a weak interaction, particularly when a neutral molecule acts as an electron donor. Thus, there is an interesting question of how to enhance carbon bonding. In this paper, we found that the ⁻OCH₃ group at the exocyclic carbon of fulvene can form a moderate carbon bond with NH₃ with an interaction energy of about -10 kJ/mol. The ⁻OSiH₃ group engages in a stronger tetrel bond than does the ⁻OGeH₃ group, while a reverse result is found for both ⁻OSiF₃ and ⁻OGeF₃ groups. The abnormal order in the former is mainly due to the stronger orbital interaction in the ⁻OSiH₃ complex, which has a larger deformation energy. The cyano groups adjoined to the fulvene ring not only cause a change in the interaction type, from vdW interactions in the unsubstituted system of ⁻OCF₃ to carbon bonding, but also greatly strengthen tetrel bonding. The formation of tetrel bonding has an enhancing effect on the aromaticity of the fulvene ring.
Keywords: aromaticity; fulvene; substituents; tetrel bonding.