Temperature-dependent crystal structures and unusual dielectric property in [Me2NH2]2[Ni(mnt)2]

The title compound bis(dimethylammonium) bis(dicyanoethene-1,2-dithiolato)-nickel (II) undergoes a reversible dielectric transition occurring at about 260 K. The variable-temperature X-ray structural determinations reveal that both phases are monoclinic, space group C2/c with a = 16.383(3), b = 9.521(5), c = 13.631(3) A, β = 118.28(3)°, V = 1872.5(8) A3 at the room temperature (293 K), and with a = 15.885(5), b = 9.418(0), c = 13.771(3) A, β = 116.89(3)°, V = 1837.4(8) A3 at the low temperature (93 K). The [Ni(mnt)2]2 − anions form a non-uniform stacking pattern and interact with the [Me2NH2]+ cations by N–H∙∙∙S, N–H∙∙∙N hydrogen bonds, for the two phases. With temperature increasing, the cations and anions are shifting and thus result in the significant changes of the hydrogen bonds. The effect of these shifts is reflected in the dielectric behavior of the [Me2NH2]2[Ni(mnt)2] salt (mnt2 − = maleonitriledithiolate).