Synthesis, Structures, Polymorphism, and Magnetic Properties of Transition Metal Thiocyanato Coordination Compounds

Reaction
of manganese, iron, and nickel thiocyanate with 4-ethylpyridine
leads to the formation of single crystals of compounds with composition
M­(NCS)2(L)4 (1), M­(NCS)2(L)2(H2O)2 (2-Mn),
and M­(NCS)2(L)2 (3) with M = Mn,
Fe, and Ni and L = 4-ethylpyridine. For most compounds, different
polymorphic modifications are observed, and their transition behavior
and thermodynamic stability was investigated. Additionally, compounds
of composition M­(NCS)2(L)2 (M = Mn or Ni) were
prepared from solution and by thermal decomposition of compounds 1 and 2, which lead to different stable and metastable
modifications. The crystal structures of most compounds were determined
by single crystal X-ray diffraction and some of them by Rietveld refinements.
Compounds 1 and 2 consist of octahedrally
coordinated discrete complexes with terminal N-bonded thiocyanato
anions. In compounds 3, the metal cations are linked
by pairs of μ-1,3-bridging thiocyanato anions into chains. Surprisingly,
thermal decomposition of Ni­(NCS)2(4-ethylpyridine)4 leads to the formation of a new compound of composition Ni­(NCS)2(4-ethylpyridine) (4-Ni). Magnetic measurements
reveal that 3-Mn/II and 3-Mn/III show antiferromagnetic
ordering at TN = 21.5 and 23.9 K and that 4-Ni is a metamagnet with a critical field of 1.4 kOe at 2
K. All other compounds show Curie or Curie–Weiss behavior with
no magnetic anomalies.