Ralph Schenker, Høgni Weihe, Hanspeter Andres, Reto Basler, Grégory Chaboussant, Kirsten Michelsen, Michael Aebersold, Herma Büttner and Hans U. Güdel
Competing Interactions in the Tetranuclear Spin Cluster {Ni[(OH)₂Cr(bispictn)]₃}I₅·5H₂O. An Inelastic Neutron Scattering and Magnetic Study
Inorg. Chem. 41, 4266-4274 (2002)                 

Abstract: The properties of the spin state manifold of the tetranuclear cluster Ni[(OH)₂Cr(bispictn)]₃}I₅·5H₂O (bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine) are investigated by combining magnetic susceptibility and magnetization measurements with an inelastic neutron scattering (INS) study on an undeuterated sample of Ni[(OH)₂Cr(bispictn)]₃}I₅·5H₂O. The temperature dependence of the magnetic susceptibility indicates an S = ¹/₂ ground state, which requires antiferromagnetic interactions both between Cr³⁺ and Ni²⁺ ions and among the Cr³⁺ ions. INS reveals potential single-ion anisotropies to be negligibly small and enables an accurate determination of the exchange parameters. The best fit to the experimental energy level diagram is obtained by an isotropic spin Hamiltonian H = J_CrNi(S₁·S₄ + S₂·S₄ + S₃·S₄) + J_CrCr(S₁·S₂ + S₁·S₃ + S₂·S₃) with J_CrNi = 1.47 cm^–1 and J_CrCr = 1.25 cm^–1. With this model, the experimental intensities of the observed INS transitions as well as the temperature dependence of the magnetic data are reproduced. The resulting overall antiferromagnetic exchange is rationalized in terms of orbital exchange pathways and compared to the situation in oxalato-bridged clusters.