Formation of 2∶1 insulating complexes of D+·D+·A2− alternating stack and a 4∶1 semimetallic complex using M(dto)2 dianions (M = Ni, Pd or Pt and dto = dithiooxalate)

The preparation of charge transfer (CT) complexes of dithiooxalate (dto) transition metal salts M(dto)2 (M = Ni, Pd, Pt or Cu) with a variety of electron donor molecules was examined based on the redox properties of constituent molecules. Conventional electron donors of TTF type, such as bis(ethylenedithio)- (BEDT-TTF (ET)) and bis(ethylenedioxy)-tetrathiafulvalene (BEDO-TTF (BO)), mainly afford 2∶1 solid CT complexes composed of a donor cation dimer D+·D+· and dianion of M(dto)2 (A2−) when M = Ni, Pd or Pt. The constituent molecules stack alternately to construct a D+·D+·A2− column. Their crystal structures are unique among the ET and BO complexes so far known, namely the M(dto)22− ions are isolated from each other being surrounded by six dimers of D+· in the crystals. Three kinds of 2∶1 complex were obtained between ET and Pd(dto)2. The electronic spectra of the solids are characterized by the particular intradimer transition of the donor cation dimer. The infrared vibrational spectra strongly exhibit the ag modes of the donor cation due to its dimerization. All of the complexes are electrical insulators. A singlet–triplet magnetic excitation is clearly observed for some of them. A highly conductive 4∶1 complex was obtained between ET and Pd(dto)2, (ET)4[Pd(dto)2], which exhibited electronic absorption extending below 5 × 103 cm−1. The complex shows a semimetallic nature.