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 [(Me2N)2CC(NMe2)2]2+[(C2N4S2)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 pi-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 pi-dimers is the closed-shell electronic configuration, although a noticeable contribution from a configuration with a singlet biradical character was also found. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).