Sonderdruckanforderungen an Prof. Dr. Dr. h.c. mult. H. Bock. Fax: +49 (0)69 798-29188. Professor Dieter Seebach gewidmet Z. Naturforsch. 55 b, 597-602 (2000); eingegangen am 21. Dezember 1999 Hexathia-dispiro[4.0.4.4]decane, Tetrahydrotetrathiafulvalenium Radical Cation Salt, DFT CalculationsThe dark red tetrahydrotetrathiafulvalenium radical cation has been generated 20 years ago by iodine oxidation of hexathia-dispiro[4.0.4.4]decane and characterized in solution by its temperature-dependent ESR spectrum. Here we report the single crystal growth of its tetrachloroaluminate salt as well as its low temperature structure determination, which proves the C=C double bond to be elongated to 140 pm length and one molecular half to be twisted by 7°. The assumed slight cyanine type distortion is supported by structural comparison with tetrakis(dimethylamino)ethene dication salts and with the 1:3 complex of tetrahydrotetrathiafulvalene mercury dichloride. Unrestricted Density Functional Theory calculations predict both the spin as well as the positive charge to be distributed predominantly within the 7r molecular center {S2C=CS2}.