Diamagnetic π‐Dimers of the [1,2,5]Thiadiazolo[3,4‐ c ][1,2,5]thiadiazolidyl Radical Anion in the Crystalline State: Preparation and X‐ray Crystal Structure of a [(Me 2 N) 2 CC(NMe 2 ) 2 ] 2+ [(C 2 N 4 S 2 ) 2 ] 2– Salt

Chemical reduction of [1,2,5]thiadiazolo[3,4‐ c ][1,2,5]thiadiazole ( 2 ) with 1,1,2,2‐tetrakis(dimethylamino)ethene ( 3 ) leads to the [ 3 ] 2+ [ 1 2 ] 2– salt ( 4 ) whose X‐ray crystal structure reveals diamagnetic π‐dimers of the [1,2,5]thiadiazolo[3,4‐ c ][1,2,5]thiadiazolidyl radical anion ( 1 ). The interplanar separation of 324.5 pm in the [ 1 2 ] 2– dimers is significantly shorter than the sum of the isotropic van der Waals radii of two S atoms of 348–358 pm. CASSCF calculations suggest that the largest contributor to the singlet ground state of these π‐dimers is the closed‐shell electronic configuration, although a noticeable contribution from a configuration with a singlet biradical character was also found. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)