Origin of the High Electrical Conductivity of Neutral [Ni(ptdt)₂] (ptdt²⁻ = propylenedithiotetrathiafulvalenedithiolate):  A Route to Neutral Molecular Metal

A novel neutral nickel complex molecule with the extended TTF dithiolato ligand, propylenedithiotetrathiafulvalenedithiolate [ptdt 2- = (S 8 C 9 H 6 ) 2- ], was synthesized. In the [Ni(ptdt) 2 ] crystal. [Ni(ptdt) 2 ] molecules form one-dimensional columns along the a axis, having short intermolecular transverse S···S contacts. The crystal exhibited an extremely high electrical conductivity (7 S cm -1 ) at room temperature as a neutral molecular crystal. High-pressure resistivity measurements were made up to 72 kbar, which revealed that the resistivity could not be suppressed by applying high pressure unlike the usual highly conducting low-dimensional organic conductors. The tight-binding band structure calculation indicated that the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) formed “crossing bands”, whose Fermi surfaces tended to vanish due to the HOMO-LUMO interactions. Only very small electron and hole pockets appeared in the Fermi surface due to the transverse interactions between neighboring columns. On the basis of these analyses, the requirements for the development of molecular metals composed of single molecules are discussed.