The benzophenone dianion [diphenyloxidomethanide, (Ph2CO)2−], which occurs in the well known deeply violet sodium/benzophenone tetrahydrofuran solutions, was crystallised with sodium cations in form of the two polymeric chain compounds [Na2(Ph2CO)(tetraglyme)]∞1 and [Na2(Ph2CO)(thf)2]∞2. It was found to aggregate with its conjugated acid, the alcoholate (Ph2CHO)−, around a central unit of sodium hydroxide, resulting in the mixed cage compound [Na13(Ph2CO)4(Ph2CHO)4(OH)(mtbe)4]·mtbe 3. The structural parameters of the benzophenone dianion indicate that a considerable amount of its negative charge is located within the phenyl rings, rather than on the formally anionic benzylic carbon atom. The topological analysis of the electron density of the monomeric model structure [Na2(Ph2CO)] reveals an even positive charge for this particular atom, hence (Ph2CO)2− is, despite its usual representation, not a vicinal dianion.
