(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, ( n Bu 4 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 n Bu 4 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°. The tight-binding band structure calculation, based on transfer integrals estimated by the extended Hückel 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 χ 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, χ follows the Curie−Weiss law: χ = C /( T − θ); C = 0.383 emu K mol -1; and θ = −4 K. Weak ferromagnetism sets in below 20 K. The coercive field is ±1.3 kOe at 2.0 K.