Pancake Bonding in π-Stacked Trimers in a Salt of Tetrachloroquinone Anion.

The crystal structure of [4-damp])2 [Cl4 Q]3 (4-damp=4-dimethylamino-N-methylpyridinium, Cl4 Q=tetrachloroquinone) salt is built up from slipped columnar stacks of quinoid rings composed of closely bound trimers with the intra-trimer separation distance of 2.84 Å and total charge of -2 whereas the inter-trimer distance is 3.59 Å. The individual rings exhibit partial negative charges that are distributed unevenly among the three Cl4 Qs in the trimer. The strong interactions within a trimer (Cl4 Q)3 2- have a partially covalent character with two-electron/multicentered bonding, that is extended over three rings, plausibly termed as "pancake bonding". The electron pairing within this multicentre bond leads to the fact that the crystals are diamagnetic and act as insulators. The studies of the structure and nature of bonding are based on X-ray charge density analysis and density functional theory.