Square-planar, anionic, paramagnetic (S = ½) nickel complexes of the 2-(trifluoromethyl)acrylonitrile-1,2-dithiolate ligand (abbreviated as tfadt) are prepared and crystallized as Et4N+ and Me4N+ salts. In the Me4N+ salt, the complex [Ni(tfadt)2]− anion adopts a trans conformation with fully ordered CF3 groups. The uniform stacking of [Ni(tfadt)2]− anions leads to the formation of a spin chain. The temperature dependence of the magnetic susceptibility shows a 2nd order phase transition to a singlet ground state below 200 K. On the other hand, in its Et4N+ salt, [Ni(tfadt)2]− exhibit a cis conformation with one ordered and one disordered CF3 groups, and a strongly dimerised spin chain structure at room temperature. Upon warming up the crystals of [Et4N][Ni(tfadt)2] above 100 °C, a 1st order phase transition with a 9 K hysteresis is observed in the temperature dependence of the magnetic susceptibility at 355–364 K. A second X-ray data collection performed on [Et4N][Ni(tfadt)2] at 102 °C (375 K) demonstrates that a structural transition to a uniform spin chain system has taken place, now with the two CF3 groups of the [Ni(tfadt)2]− anion strongly affected by disorder. The 1st order character of the structural transition observed in the Et4N+ salt is tentatively attributed to the ordering of one CF3 group, by comparison with the Me4N+ salt which exhibits no such disorder and a 2nd order phase transition to the singlet ground state.
