Synthesis, Crystal Structure, and Physical Properties of (BEDT-TTF)[Ni(tdas)₂] (BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene; tdas = 1,2,5-Thiadiazole-3,4-dithiolate): First Monomeric [Ni(tdas)₂]^- Monoanion

We report the synthesis, structure, and physical properties of (BEDT-TTF)[Ni(tdas)2] [BEDT-TTF, or ET, is bis(ethylenedithio)tetrathiafulvalene; tdas is 1,2,5-thiadiazole-3,4-dithiolate], which is the first example of a salt containing monomeric [Ni(tdas)2]- monoanions. This salt, which crystallizes in the monoclinic space group P21/c with a = 17.2324(6) Å, b = 13.2740(5) Å, c = 10.9467(4) Å, β = 96.974(2)°, and V = 2485.5(2) Å3, forms a layered structure. One layer contains dimerized BEDT-TTF electron donor molecules and isolated [Ni(tdas)2]- monoanions, while the second layer contains chains of [Ni(tdas)2]- monoanions. Conductivity measurements show that (BEDT-TTF)[Ni(tdas)2] has a semiconductor-to-semiconductor transition near 200 K, while magnetic measurements indicate that it is an S = 1/2 paramagnet with weak antiferromagnetic coupling. Reflectance spectra reveal bands in the near-infrared region (6.6 × 103 and 10.6 × 103 cm-1) which are typical of (BEDT-TTF)22+ dimers. From these data, we can conclude that the unpaired electron lies on the [Ni(tdas)2]- anions. Tight-binding band structure calculations were used to analyze the electronic structure of this salt.