(NBu₄)Ni(mnt)₂: A novel bistable high temperature spin-Peierls-like system

The title compound, tetrabutylammonium bis(1,2-dimercaptoethene-1,2- dicarbonirtilato-S,S′)-nickel (III) [henceforth (NBu₄)Ni(mnt) ₂] undergoes a hysteretic phase transition that occurs at 172 K upon cooling. Crystallographic studies show that the phase transition is an order-disorder type involving one arm of the NBu4+ cation. Both phases are monoclinic, space group C2/c with a = 30.860(3), b = 13.804(2), c = 15.678(2) Å and β = 115.642(3)° with V = 6020.8(11) ų at room temperature and with a = 31.073(3), b = 13.928(1), c = 14.414(1) Å and β = 115.073(2)° and V = 5652.1(9) ų at 84 K. In both phases, the anions form non-uniform stacks parallel to the crystallographic c direction. In the high temperature phase, the Ni-S distances between anions are accidentally equal, and the stacks behave as nearly uniform antiferromagnetic chains with nearly equal exchange constants. At the phase transition, a non-uniform compression of the stacks occurs and the magnetic moment drops rapidly, indicating a magnetic dimerization of the stacks. Susceptibility and C_p measurements show an approximate 7°C thermal hysteresis.