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-kappa N)(2)(NH3)(4)] has been characterized in the solid state by X-ray and neutron diffraction at temperatures ranging from ii 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, a lthough then is reason to believe that at a finer level of distinction it i s really Cmcm approximating Fmmm. The nickel center lies on a site of appar ent point symmetry mmm. At lower temperatures, the space group is unambiguo usly Cmcm without appreciable change in the unit cell parameters. and the m olecule lies at a site of m2m symmetry. The shape of the molecule changes s moothly 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 cause s of this phenomenon are discussed. The increase in intensity on cooling of some of the bands observed in the polarized crystal spectrum of the comple x is consistent with the change in the molecular structure. Bonding paramet ers derived from the transition energies indicate that the cyanurate produc es 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-We iss 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.