The molecular structure and the conformational composition of fumaryl fluoride were determined by low‐temperature vibrational spectroscopy and single‐crystal X‐ray structure analysis. Three planar rotational isomers, trans‐trans‐, cis‐cis‐ and cis‐trans‐fumaryl fluoride were identified. C4H2F2O2 crystallizes in the monoclinic space group P21/c with four formula units per unit cell. Besides, Lewis acid‐base adducts between fumaryl fluoride and arsenic pentafluoride were synthesized. These adducts, which contain O–As bonding interactions, were found to crystallize as the monoadducts trans‐cis‐C4H2F2O2·AsF5 and cis‐trans‐C4H2F2O2·AsF5. Moreover, the diadduct trans‐trans‐C4H2F2O2·2 AsF5 was determined by X‐ray crystallography. The experimental data are discussed together with quantum chemical calculations of trans‐trans‐, cis‐cis‐, and cis‐trans‐fumaryl fluoride.

The reaction of fumaryl fluoride with the superacidic solutions XF/MF5 (X=H, D; M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C4H3F2O2]+[MF6]− (M=As, Sb) and [C4H2X2F2O2]2+([MF6]−)2 (X=H, D; M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low‐temperature vibrational spectroscopy. Low‐temperature NMR spectroscopy and single‐crystal X‐ray structure analyses were carried out for [C4H3F2O2]+[SbF6]− as well as for [C4H4F2O2]2+([MF6]−)2 (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C4H4F2O2 ⋅ 2 HF]2+ and [C4H3F2O2 ⋅ HF]+ at the B3LYP/aug‐cc‐pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge‐related properties of the diprotonated species. The C−F bond lengths in the protonated dication are considerably reduced on account of the +R effect.

The C−F bond of fumaryl fluoride is strengthened by protonation of the oxygen atom in a sea of superacid. The chain represents the reinforced connection between the carbon atom and the fluorine atom. The C−F bond even reveals a slight double‐bond character and a formal positive charge on the fluorine atom. More information can be found in the Research Article by A. J. Kornath, et al. (DOI: 10.1002/chem.202104422).

The cover picture shows the solid architectural construction of three stones, with each stone illustrated with the molecular structures of fumaric acid and its deprotonated and O,O'‐diprotonated species. Fumaric acid was investigated in the superacidic solutions HF/SbF5 and HF/AsF5. The salts of the diprotonated and the monoprotonated fumaric acid were isolated and characterized. The carbon‐oxygen skeleton remains unaltered regardless of protonation or deprotonation. The stability of the carbon‐oxygen skeleton is represented by the stones built on top of each other. More details can be found in the article by Marie C. Bayer, Christoph Jessen, and Andreas J. Kornath on page 333 ff.

Fumaric acid was reacted with the binary superacidic systems HF/SbF5 and HF/AsF5. The O,O'‐diprotonated [C4H6O4]2+([MF6]–)2 (M = As, Sb) and the O‐monoprotonated [C4H5O4]+[MF6]– (M = As, Sb) species are formed depending on the stoichiometric ratio of the Lewis acid to fumaric acid. The colorless salts were characterized by low‐temperature vibrational spectroscopy. In case of the hexafluoridoantimonates single‐crystal X‐ray structure analyses were carried out. The [C4H6O4]2+([SbF6]–)2 crystallizes in the monoclinic space group C2/c with four formula units per unit cell and [C4H5O4]+[SbF6]– crystallizes in the triclinic space group P1 with one formula unit per unit cell. The protonation of fumaric acid does not cause a notable change of the C=C bond length. The experimental data are discussed together with quantum chemical calculations of the cations [C4H6O4 · 4 HF]2+ and [C4H6O4 · 2 H2CO · 2 HF]2+.