Attraction Between Like Charged Ions in Ionic Liquids: Unveiling the Enigma of Tetracyanoborate Anions

Intermolecular interactions in ionic liquids are mainly governed by Coulomb forces. Attraction between cations has been previously observed and was attributed to dispersion interactions between non-polar moieties, hydrogen bonding, or π-stacking. In this study, we present the intriguing behaviour of tetracyanoborate anions in ionic liquids that, unlike their dicyanamide and tricyanomethanide counterparts, form dimers in both solid and liquid phases. A joint simulation and experimental study uncovers the origin of such anion-anion attraction: stabilisation by induction and dispersion forces between several cyano-groups, which is strong enough to overcome electrostatic repulsion. These findings open up new opportunities in the rational design of ionic liquids, where interactions between ions of the same charge can be controlled and fine-tuned by the presence of cyano-groups.