(nBu4N)[Ni(dmstfdt)2]:: A planar nickel coordination complex with an extended-TTF ligand exhibiting metallic conduction, metal-insulator transition, and weak ferromagnetism

The 1:1 tetrabutylammonium salt of a nickel complex with an extended-TTF ligand, ((Bu4N)-Bu-n)[Ni(dmstfdt)(2)] (dmstfdt = dimethyldiselenadithiafulvalenedithiolate) (1) is a unique ambivalent molecular system exhibiting weakly metallic conduction above room temperature and a weak ferromagnetism at low temperature. Its X-ray structure consists of two independent [Ni(dmstfdt)(2)](-) (A and B) and two (Bu4N+)-Bu-n in the unit cell. The pseudo-planar anions are arranged in a zigzag -ABA'B'- manner along the molecular side-by-side direction with a dihedral angle of the molecular planes of 42.6 degrees. The tight-binding band structure calculation, based on transfer integrals estimated by the extended HUckel approximation, presents both three-dimensional electron and hole Fermi surfaces, which is consistent with the observed weakly metallic conduction around room-temperature despite the 1:1 stoichiometry of the complex. At 147 K, 1 shows a sharp insulating transition associated with the localization of one electron on each nickel complex below 160 K. The chi T values of 1 increase linearly from 0.129 to 0.383 emu K mol(-1) with decreasing temperature (160-340 K), suggesting a gradual electron localization with lowering the temperature. In the range of temperature of 80-150 K, chi follows the Curie-Weiss law: chi = C/(T - theta); C = 0.383 emu K mol(-1); and theta = -4 K. Weak ferromagnetism sets in below 20 K. The coercive field is +/- 1.3 kOe at 2.0 K.