Synthesis, structure and properties of [ethylpyridinium][Ni(mnt)(2)]: evidence for an unusual magnetically ordered ground state

[Ethylpyridinium][Ni(mnt)(2)] has been prepared by electrochemical oxidation of the corresponding dianionic salt. The X-ray structure has been determined. Three different environments were observed for the paramagnetic anions with two in a stacking mode and one approximately orthogonal to the stack. The stacking showed an ACA-type repeat unit with an inversion centre on complex C. Magnetic susceptibility measurements show Curie-Weiss behaviour with a change in parameters at around 50 K. Above this temperature a Curie constant of 0.3 emu K mol(-1) and a Weiss constant of -150 K were observed and below 50 K these values were 0.08 emu K mol(-1) and +2 K respectively. A maximum in the susceptibility was observed at 285 mK which suggests low temperature ordering to a phase with a non-cancelled alignment of the spins. Intermolecular interactions and the effect of temperature were assessed in terms of transfer integral calculations.