Physical properties and electronic states of cation radical salts based on
unsymmetrical type metal-dithiolene complexes
1Kazuya Kubo , Akiko Nakao2, Yasuyuki Ishii1, Takashi Yamamoto1, Masafumi Tamura1, Reizo
Kato1, Kyuya Yakushi3 and Gen-etsu Matsubayashi4
1. RIKEN, JST-CREST, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
2. KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.
3. Institute of Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
4. Department of Molecular Chemistry, Graduate School of Engineering, Osaka University,2-1, Yamadaoka, Suita, Osaka 565-0871, Japan.
Recently, we have succeeded in preparation and crystallization of various radical cation salts
based on unsymmetrical metal-dithiolene complexes [(bpy or ppy)M(S-S)]2[Q][solvent]n [bpy =2,2'-bipyridine; ppy- = C-deprotonated-2-phenylpyridine; S-S = C8H4S82- and C8H4S6O22-; Q =PF6-, BF4-, AsF6- and TaF6-; solvent = PhCl and PhCN; n =0-0.5].1-3 Among them, the salt[(ppy)Au(C8H4S8)]2[PF6] (1) (Figure 1) is the first metallic molecular conductor based on theorgamometallic and unsymmetrical metal-dithiolene complexes, even though pressure above
0.8 GPa is required to achieve this state. In the crystal, the columnar structure is formed by
two-fold head-to-head stacking of the cation radicals. Energy band calculation based on thecrystal structure indicated that the salt has a quasi-one dimensional Fermi surface. However,the salt is an insulator at ambient pressure (ρr.t. = 2.6 Ω cm; Ea = 0.03 eV). A metallic behavior
is observed from 300 K to 100 K under 0.8 GPa, and it is retained down to 20 K under 1.6GPa. In this work, in order to determine the ground state of the salt exactly, magneticsusceptibility, Raman and infrared reflectance spectra were measured. The spin susceptibilityof 1 gradually decreased from 300 K (χ = 5.1×10cm3 mol-1) to 50 K (χ = 4.0×10-4 cm3mol-1). Below 50 K, the susceptibility exhibited a rapid decrease (χ = 2.8×10-4 cm3 mol-1 at 12K) accompanied by anisotropic temperature dependence, which is an indication of an-4antiferromagnetic ordering. Raman spectra of 1 showed three peaks (1520, 1490, 1460 cm-1)
which are due to three C=C double bonds on the ditholene ligand. These peaks do not shift byvariation in temperature (300 K -10 K), indicating that the molecular charges do not change
from 300 K to 10 K. In the infrared reflectance spectrum of 1, electronic contribution between
donors was observed around 2500 cm-1 for E || a, not shown for E || b and c. The spectrum
along the a axis also exhibited a broad vibronic peak (1100 cm-1) which is explained by thecharge transfer transitions occurring in a dimeric structure. Taking the results into account, weconcluded that the salt has the quasi-one dimensional electronic character and the groundstate is a Mott insulating state close to the metal-insulator boundary under ambient pressure
with +1 of formal charge on the dimer.
Other radical cation salts including [(ppy)Au(C8H4S6O2)]2[BF4] and [(bpy)Pt(C8H4S8)][BF4](Figure 1) have similar molecular arrangements of 1. In spite of the similarity, their conducting
properties (ρr.t. > 105 Ω cm) and electronic states were quite different from those of 1. We will

also discuss their conducting and other physical properties.
1. K. Kubo et al., Synth. Met. 2005, 153, 425-428.
2. K. Kubo et al., Eur. J. Inorg. Chem. 2003, 4093-4098.
3. K. Kubo et al., Inorg. Chim. Acta 2002, 336, 120-124.

Figure 1. Schematic drawing of unsymmetrical donor molecules.