Origin of the high electrical conductivity of neutral [Ni(ptdt)(2)] (ptdt(2-) = propylenedithiotetrathiafulvalenedithiolate): A route to neutral molecular metal

A novel neutral nickel complex molecule with the extended TTF dithiolato li gand. propylene-dithiotetrathiafulvalenedithiolate [ptdt(2-) = (S8C9H6)(2-) ], was synthesized. In the [Ni(ptdt)(2)] crystal. [Ni(ptdt)(2)] molecules f orm one-dimensional columns along the a axis, having short intermolecular t ransverse S ... S contacts. The crystal exhibited an extremely high electri cal conductivity (7 S cm(-1)) at room temperature as a neutral molecular cr ystal. High-pressure resistivity measurements were made up to 72 kbar, whic h revealed that the resistivity could not be suppressed by applying high pr essure unlike the usual highly conducting low-dimensional organic conductor s. The tight-binding band structure calculation indicated that the HOMO (hi ghest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) formed "crossing bands", whose Fermi surfaces tended to vanish du e to the HOMO-LUMO interactions. Only very small electron and hole pockets appeared in the Fermi surface due to the transverse interactions between ne ighboring columns. On the basis of these analyses, the requirements for the development of molecular metals composed of single molecules are discussed .