Partially Covalent Two-Electron/Multicentric Bonding between Semiquinone Radicals

X-ray charge density was determined and analyzed for two polymorphs of the N-methylpyridinium salt of the tetrachlorosemiquinone radical anion and its analogous closed-shell relatives, tetrachloroquinone (chloranil) and tetrachlorohydroquinone. The study, which was combined with calculations of electron delocalization, electrostatic potentials, and aromaticity, presents details of electronic structure of the semiquinoid ring. This comparative study reveals that the negative charge is delocalized over the entire semiquinone radical and that the chlorine substituents play a crucial role in its stabilization through induction effect. In general, the semiquinoid ring has partially delocalized π-electrons and is approximately halfway between a quinoid and an aromatic ring. In the orthorhombic polymorph with stacks of equidistant radicals electron density between the rings of almost 0.05 e Å^–3 and four (3,–1) saddle points between the contiguous rings were found. In the diamagnetic triclinic polymorph, comprising strongly bound radical dimers (with significant covalent character—“pancake bond”), maximum electron density between the rings exceeds 0.095 e Å^–3, and multiple (3,–1) critical points are found. However, only negligible electron density is observed between the dimers. Thus, in the radical anion stacks spin coupling, along with dispersive and polarization effects, defines interplanar distance and magnetic behavior, whereas intermolecular electrostatic potential determines the ring offset.