Tunable molecular distortion in a nickel complex coupled to a reversible phase transition in the crystalline state

The six-coordinate coordination complex trans-[Ni(cyan-xN)2(NH3)4] has been characterized in the solid state by X-ray and neutron diffraction at temperatures ranging from 11 to 298 K, by electronic spectroscopy over the temperature range 14-297 K, and by magnetic susceptibility measurements from 1.8 to 300 K. At room temperature the observed space group is Fmmm, although there is reason to believe that at a finer level of distinction it is really Cmcm approximating Fmmm. The nickel center lies on a site of apparent point symmetry mmm. At lower temperatures, the space group is unambiguously Cmcm without appreciable change in the unit cell parameters, and the molecule lies at a site of m2m symmetry. The shape of the molecule changes smoothly with temperature variations from room temperature down to about 140 K, in a behavior characteristic of second-order phase transformations. The molecular shape varies, but by lesser amounts, below 140 K. Possible causes of this phenomenon are discussed. The increase in intensity on cooling of some of the bands observed in the polarized crystal spectrum of the complex is consistent with the change in the molecular structure. Bonding parameters derived from the transition energies indicate that the cyanurate produces a very weak ligand field, which is consistent with the long metal-ligand bond to this ligand. The magnetic properties of the solid display Curie-Weiss behavior through the temperature range of the most pronounced molecular shape changes, but antiferromagnetic interactions become significant below 50 K, with antiferromagnetic ordering at 2.61 K. The propagation pathways for the magnetic interactions are inferred.