A highly labile dimer of hydrogen cyanide, HCN⋅⋅⋅HCN, was extracted from liquid HCN by adduct formation with the bulky Lewis acid B(C F ) , affording HCN⋅⋅⋅HCN-B(C F ) , which was fully characterized. The influence of the solvent (HCN, CH Cl , and aromatic hydrocarbons) on the crystallization process was studied, revealing dimer formation when using HCN or CH Cl as solvent, whereas aromatic hydrocarbons led to the formation of monomeric arene⋅⋅HCN-B(C F ) adducts, additionally stabilized by η -coordination of the aromatic ring system similar to well-known half-sandwich complexes.