Magneto-structural correlations in one-dimensional Ni(en)_2Pd(CN)_4: magnetic properties and redetermination of the crystal structure at two temperatures

摘要

The magneto-structural correlations in Ni(en)_2Pd(CN)_4 (NEPC) have been studied. The crystal structure is formed by infinite electroneutral 2,2-TT type chains [-Ni(en)_2-NC-Pd(CN)_2-CN-] (en = 1,2-diaminoethane) in which the paramagnetic nickel atoms are bridged by diamagnetic tetracyanopalladate anions. Contrary to the precious room temperature X-ray analysis of NEPC, the results of the current structure determinations at both 293 and 150 K showed the nickel atom to be coordinated in the form of slightly elongated octahedron with two chelate bonded and disordered en molecules in the equatorial plane (mean Ni-N distance of 2.096 A at both temperatures) and N-bonded bridging cyano groups in the axial positions (Ni-N distances of 2.117 and 2.108 A at 293 and 150 K, respectively). Consequently, the zero field splitting parameter D is finite and positive. The chains are held together by weak hydrogen bonds of the N-H…N(≡C) type. Magnetisation, susceptibility and specific heat measurements indicate that NEPC represents an S = 1 Heisenberg magnetic chain characterised by strong planar anisotropy, D/k_B ≈ 5 K, weak in-plane anisotropy, E/D = 0.1, and by a small exchange coupling constant, J/k_B ≈ 0.25 K. These properties are similar to those exhibited by an analogous compound Ni(en)_2Ni(CN)_4.