Hal center dot center dot center dot Hal interactions in a series of threeisostructural salts of halogenated tetrathiafulvalenes. Contribution of the halogen atoms to the HOMO-HOMO overlap interactions

The halogenated tetrathiafulvalenes, Br-2-EDT-TTF and I-2-EDT-TTF were prep ared by the coupling route from the corresponding 4,5-dibromo- or 4,5-diiod o-1,3-dithiole-2-thione. In the isostructural series, (Br-2-EDT-TTF)(2)IBr2 , (Br-2-EDT-TTF)(2)I-3 and (I-2-EDT-TTF)(2)I-3, noted Br-2/IBr2, Br-2/I-3 a nd I-2/I-3 respectively, short (3.42-3.60 Angstrom) and directional Hal . . . Hal interactions are identified between donor molecules and with the ani ons, stabilising rare beta' phases. These interactions play not only a stru ctural role but also contribute to the electronic dispersion thanks to size able coefficients on the halogen atoms in the HOMOs of Br-2-EDT-TTF and I-2 -EDT-TTF. The three salts behave as Mott insulators as reflected by a high room temperature conductivity (0.5 S cm(-1)) with an activation energy whic h increases in the order Br-2/IBr2 (730 K), Br-2/I-3 (1260 K), I-2/I-3 (133 0 K) and a weak magnetic susceptibility, which decreases abruptly below 150 K with no sign of an antiferromagnetic ground state.