Crystal structures and magnetic properties of the interpenetrating rutile-related compounds M(tcm)2 [M = octahedral, divalent metal; tcm– = tricyanomethanide, C(CN)3–] and the sheet structures of [M(tcm)2(EtOH)2] (M = Co or Ni)

Abstract

The isomorphous structures of M(tcm)₂ [M^II = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd or Hg; tcm^– = tricyanomethanide, C(CN)₃^–] contain two interpenetrating rutile-related networks generated by octahedral six-connecting metal ions and trigonal three-connecting tcm^– anions. The detailed variable temperature and variable field magnetic properties of this series of high-spin complexes generally point to the existence of very weak intraframework coupling with no evidence for long range magnetic order or interframework effects. The compound Cr(tcm)₂ is the most strongly coupled and displays a field independent maximum in susceptibility at 14.5 K and a J value of –1.6 cm^–1 (using a –2JS₁·S₂ Heisenberg chain model); Co(tcm)₂ displays high temperature magnetic moments typical of essentially uncoupled octahedral centres but with a most unusual field dependence in µ_Co observed below 10 K, probably due to very weak ferromagnetic coupling, and an M_sat value in high fields, at 2 K, which is significantly less than that expected for S = 3/2. Doping of M(tcm)₂ with another M′ member of the series leads to a crystal structure isomorphous with M(tcm)₂ in which the dopant metal M′ occupies the M site in a random fashion. The resultant magnetism is simply intermediate between those of the parent phases. The isomorphous structures of [M(tcm)₂(EtOH)₂] (M^II = Co or Ni) contain pseudo square-grid sheets in which the tcm^– ligands are each co-ordinated to two metal ions and act as a kinked bridge. Each metal is co-ordinated to four tcm^– anions in an equatorial arrangement and to two axial ethanol ligands. Extensive intrasheet hydrogen bonding exists between the ethanol molecules and the unco-ordinated nitrile of the tcm^– bridges.