Since the report of the first organic superconductor TMTSF 2 PF 6 (TMTSF ¼ tetramethyltetraselenafulvalene), [1] various types of molecular metals have been synthesized.[2] The most intensively studied molecular conductors are D 2 X-type molecular metals, where D is a p-donor molecule, such as TMTSF and BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene), and X is a counter anion, such as PF 6 À and I 3 À . In the crystals of these conductors, the p-donor molecules form one-dimensional columns or twodimensional layers, and the counter anions, which do not contribute to the formation of conduction bands, are located between them. Consequently, in contrast to the inorganic metals consisting of typical metal elements, such as Cu and Au, almost all molecular metals synthesized until recently have been one-or two-dimensional conductors. Thus, their resistivity anisotropy is very large. In addition, these materials are not malleable like Cu and Au, and metal plates (or thin metal films) of molecular conductor are difficult to obtain. Nevertheless, few examples of thin films showing metallic conductivity down to the liquidhelium temperature have been reported. Polycarbonate films containing molecularly dispersed BEDT-TTF, when treated with I 2 (or Br 2 )/CH 2 Cl 2 , exhibit metallic properties. [3,4] Furthermore, annealed BEDT-TTF/I films have been found to exhibit superconducting behavior. It has been reported that metallic (even superconducting) thin films of TTF[Ni(dmit) 2 ] 2 (dmit ¼ 1,3-dithiole-2-thione-4,5-dithiolate) can be prepared on Si substrates by electrodeposition. [5,6] Although the methods used for the preparation of these films are highly ingenious, a much simpler method would be possible if a fine metallic powder of molecular conductors could be obtained. However, molecular metals generally exhibit nonmetallic behavior in the polycrystalline state, at least at low temperatures, due to the low-dimensionality effect and grain-boundary effect. In addition, the crystals of molecular conductors tend to be mostly thermally unstable in air, especially in the fine-powder state. However, these problems can be resolved to some extent by using single-component molecular metals.The first single-component molecular metal [Ni(tmdt) 2 ] (tmdt ¼ trimethylenetetrathiafulvalenedithiolate; Scheme 1) was prepared in 2001. [7] Since then, various types of singlecomponent molecular conductors have been synthesized. [8][9][10] In contrast to D 2 X-type molecular conductors, in which carriers are generated by the charge transfer between D and X, in single-component molecular conductors metal electrons are automatically generated by the self-assembly of a single type of molecule. The existence of the three-dimensional Fermi surfaces of [Ni(tmdt) 2 ] has been confirmed by ab initio band-structure calculations, [11] and the observation of de Haas-van Alphen oscillations at very high magnetic fields and low temperatures. [12] We have also found that the paramagnetic metallic state of isostructural [Au(tmdt) 2 ] transforms into an an...
