LOW-TEMPERATURE CRYSTAL-STRUCTURE OF THE ORGANIC METAL ([H-2(8)]BEDT-TTF)(4)CL-2-CENTER-DOT-6D(2)O [BEDT-TTF = BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE]

([H-2(8)]BEDT-TTF)(4)Cl-2 . 6D(2)O [BEDT-TTF = bis (ethylenedithio) te trathiafulvalene] exhibits a transition from semimetal to semiconducto r at T=160 K (Rosseinsky et nl., J. Mater. Chem., 1993, 3, 801). This electronic transition is accompanied by a structural transition that i s characterized by the reversible appearance of superstructure reflect ions corresponding to the doubling of the cell parameter b and a chang e in space group from Pcca to Pbcn. The crystal structures are very si milar above and below T and the calculated intermolecular transfer int egrals scarcely change. In contrast, from the difference in intramolec ular bond lengths, it is clear that the two crystallographically indep endent BEDT-TTF molecules carry different charges at low temperature, suggesting that a degree of ionicity arising in the BEDT-TTF layer is responsible for the change in electrical behaviour.