Structure and Magnetic Ordering of M^II[N(CN)₂]₂ (M = Co, Ni)^†

The reaction of [M^II(OH₂)₆](NO₃)₂ (M = Co, Ni) and [N(CN)₂]^- leads to formation of isomorphous M[N(CN)₂]₂ [M = Co (2a), Ni (3)], respectively, while the reaction of [Co^II(OH₂)₆](NO₃)₂ in 1% pyridine (py) solution with [N(CN)₂]^- leads to the formation of Co[N(CN)₂]₂py₂. The structure of 2a, α-Co[N(CN)₂]₂, was determined from the Rietveld analyses of both powder X-ray (synchrotron) and neutron as well as single-crystal X-ray diffraction data, whereas the structure of 3 was determined from the Rietveld analysis of powder neutron diffraction data. 2a (pink) and 3 (light blue) belong to the orthorhombic space group Pnnm with Z = 2 [2a (single crystal):  a = 5.9985(15), b = 7.0711(18), c = 7.4140(19) Å, V = 314.47(14) ų, R₁= 0.027. 3 (powder neutron diffraction):  a = 5.97357(25), b = 7.03196(28), c = 7.29424(22) Å, V = 306.40(3) ų, χ² = 1.650]. Thermolysis of Co[N(CN)₂]₂py₂ leads to intensely blue β-Co^II[N(CN)₂]₂, 2b. The M in 2a and 3 are six-coordinate and bound to six different μ₃-bonded [N(CN)₂]^- ligands, forming a rutile-like 3-D framework. Both M^II sites are slightly tetragonally elongated, with average axial M−N distances (295 K) of 2.161(5) (2a) and 2.137(2) Å (3) and average equatorial M−N distances of 2.091(4) (2a) and 2.051(1) Å (3), and each M^II is coordinated to four equatorial [N(CN)₂]^- ligands that bridge between two adjacent M^II sites, generating ribbon-like 1-D chains that propagate along the c-axis. Adjacent chains pack out-of-registry, with the central N2s bridging to M^II ions of adjacent chains, forming a 3-D network. [N(CN)₂]^- has pseudo-C_2v symmetry with average C1⋮N1 distances of 1.158 (2a) and 1.159 Å (3) at room temperature and C1−N2 distances of 1.315 (2a) and 1.313 Å (3). The structure of 2b has not yet been elucidated, but on the basis of the color and magnetic properties, it is thought to be comprised of tetrahedral Co(II). The shortest M···M separations are 5.936 (2a) and 5.881 Å (3) at room temperature via neutron diffraction studies. The susceptibility for 2a, 2b, and 3 can be fit by the Curie−Weiss expression with g = 2.60, θ = 9 K (T > 50 K); g = 2.27, θ = −7 K (T > 60 K); and g = 2.20, θ = 21 K (T > 50 K), respectively. The observed room-temperature effective moments of 5.13, 4.37, and 3.17 μ_B, respectively, exceed the spin-only moments as expected for these metal ions, but are consistent with octahedral Co(II), tetrahedral Co(II), and octahedral Ni(II), respectively. Ferromagnetic behavior is suggested for 2a and 3 from the 5 K, 5.5 T saturation magnetizations of 14 000 (2a) and 11 900 emu Oe/mol (3), hysteresis loops with coercive fields, H_cr, of 800 (2a), 680 (2b), and 7000 Oe (3) at 2 K, field-cooled and zero-field-cooled low-field M(T) data showing magnetic ordering below essentially magnetic field-independent bifurcation temperatures of 9.2 (2a) and 20.6 K (3), and observations of both in-phase, χ‘(T), and out-of-phase, χ‘‘(T), components of the ac susceptibility maxima slightly below the bifurcation temperatures. The T_cs are estimated from the peaks in the 10 Hz χ‘(T) data to be 8.7, 8.9, and 19.7 K for 2a, 2b, and 3, respectively. Using mean-field theory the spin-coupling energies, J/k_B, were estimated to be 0.45, −0.7, and 2.0 K for 2a, 2b, and 3, respectively. 2b shows canted antiferromagnetic behavior below its ordering temperature of 8.9 K. A second transition at 2.7 K, as observed in ac susceptibility, specific heat, and field-cooled/zero-field-cooled magnetization studies, suggests the possibility of an initially canted antiferromagnetic ground state with a change in canting angle below 2.7 K.