Complexes of bis(hexafluoroacetylacetonato)copper(II) [Cu(hfac)(2)] ligated with 3- and 4-(N-oxyl-tert-butylamino)pyridines (3NOPy and 4NOPy) and N-{3- and 4-(N-oxyl-tert-butylamino)phenyl}imidazoles (3NOIm and 3NOIm) were prepared. The 1:2 complexes [Cu(hfac)(2)(3NOPy and 4NOPy)(2)] have hexacoordinated octahedral structures in which the two pyridyl nitrogen atoms are coordinated to the copper(II) ions in the trans configuration. The 1:1 complex [Cu(hfac)(2)(4NOPy)] has a distorted pentacoordinated pseudo-head-to-tail cyclic dimer structure in which the oxygen atom of the aminoxyl group of one complex is situated at a distance of 2.79 Angstrom from the copper ion of the other complex. The magnetic properties of these and two other complexes, [Cu(hfac)2(3NOIm and 4NOIm)(2)], were investigated using a SQUID susceptometer. Temperature dependence studies of the chi(mol)T values of [Cu(hfac)(2)(4NOPy)2] revealed that the two aminoxyl radicals interact ferromagnetically with the copper(II) ion (S = 1/2) with an exchange parameter J/k(B) = 60.4 +/- 3.3 K to produce a quartet ground state (S = 3/2). On the other hand, the chi(mol)T value of the isomeric [Cu(hfac)(2)(3NOPy)(2)] was nearly constant at 0.5 emu K mol(-1) in the temperature range 5-300 K. A rectangular four-spin model was fitted to the chi(mol)T vs T plot for complex [Cu(hfac)(2)(4NOPy)] to give J/k(B) = 58.6 and 58.5 K. Similarly, the J values for [Cu(hfac)(2)(3NOIm and 4NOIm)(2)] were negative and positive, respectively, but their absolute J values were too small to determine in both cases.