A Double Bond Between Two Molecules

π-Conjugated molecules with one electronic spin often form π-stacked dimers via molecular-orbital interactions between the two unpaired electrons. These interactions are recognized as intermolecular single bonds between two π-conjugated systems. This then raises the question if a double bond between two such molecules is possible. To answer this question, we have synthesized an antiaromatic Ni(II) bis(pentafluorophenyl)norcorrole. Its dimer adopts a face-to-face-stacked structure with an extremely short stacking distance of 2.97 Å. Importantly, an intermolecular double bond does indeed exist in this system and this bond involves four bonding electrons between the two molecules. The bonding electrons were experimentally observed via synchrotron X-ray diffraction analysis and corroborated by theoretical calculations. The intermolecular interaction of the molecular orbitals imparts the stacked dimer with an aromatic character that is distinctly different from that of its monomer.