Exchange interactions in tropylium bis(1,2-dicyanoethylene dithiolato) nickelate (III)

Abstract

The compound tropylium bis[1,2‐dicyanoethylenedithiolato] nickelate (III), i.e., (C₇H₇)⁺ [Ni(mnt)₂]⁻, has been prepared and its crystal structure determined; the space group is P21/c, Z=2, the unit cell axes are a=6.364, b=7.458, c=17.991 Å, and β=91.22 °. The Ni(mnt)2− and C₇H₇+ ions form segregated stacks in the direction of the b axis. This is in contrast with the structure of the analogous thiete complex (C₇H₇)⁺ [NiS₂C₂ (CF₃)₂]⁻, in which crystals alternating stacks of cations and anions are present. The susceptibility appears to be dominated by a strong one‐dimensional, isotropic, antiferromagnetic exchange coupling (J₀=−4.4 cm⁻¹). However, the angular dependence of the EPR linewidth at room temperature has to be explained with an additional anisotropic exchange interaction of 0.26 cm⁻¹. Furthermore, Q‐band EPR measurements in the ab plane reveal a very small interstack exchange interaction (0.015 cm⁻¹ at room temperature) which increases by a factor of 4 upon lowering the temperature to 8 K and which might be responsible for a possible long range order at low temperature.