Abstract

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 a_g 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.