Three cobalt(II) complexes, [Co(X) 2(4NOpy) 4] (X = NCO -, NCS -, and Br -; 4NOpy = 4-(N-tert-butylaminoxyl)-pyridine) were prepared, and their molecular structures were characterized by X-ray structure analysis. The molecular geometry of [Co(X) 2(4NOpy) 4] (X = NCO - and NCS -) is a compressed octahedron, in which the counter ions occupy the apical positions with short bond distances of 2.064-2.098 Å. In the crystalline state, from plots of χ mol vs T and χ′ mol vs T, [Co(NCO) 2(4NOpy) 4] and [Co(NCS) 2(4NOpy) 4] are antiferromagnets with T N = 4.5 and 15 K, respectively. In frozen solution, on the other hand, both complexes and [Co(Br) 2(4NOpy) 4] functioned as single-molecule magnets. The χ″ mol vs T plot for [Co(Br) 2(4NOpy) 4] gave a effective activation barrier (U eff) of 20 K for the reorientation of the spin. From the field dependence of magnetization at various temperatures below 5 K for [Co(X) 2(4NOpy) 4] (X = NCO -, NCS -, and Br -) the values of the zero-field splitting parameters, D/k B, were estimated to be -14, -9.7, and -4.5 K with S = 5/2, respectively. Theoretical studies based on the ligand-field theory model for [Co(NCO) 2(4NOpy) 4] gave an exchange coupling parameter, J/k B, of 29 K and a thermodynamic activation barrier, U, of 60 K.
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