The nitroxide free radical 2-(3-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide (L) reacts with Cu(hfac)(2), leading to a complex of formula [Cu(hfac)(2)](4)L(2). This complex has a cyclic molecular structure where two copper ions are bridged by the free radicals. The two remaining metal centers are extracyclic. Both oxygen atoms of the nitronyl nitroxide ligand are axially bound to both types of copper(II) ions at room temperature while, at 50 K, coordination of the NO group to the intracyclic metal ion is equatorial. The magnetic behavior is strongly correlated to the change of coordination. While at room temperature one observes a Curie law corresponding to six independent S = 1/2 spins, at low temperature only two S = 1/2 spins are observed. This behavior is rationalized in the frame of the dependence of the metal-ligand exchange interaction upon the coordination geometry in nitroxide-copper derivatives. This spin transition is further characterized by EPR spectroscopy and heat capacity measurements. Relevant structural parameters at room temperature are as follows : space group , a = 11.509(2) Angstrom, b = 12.673(2) Angstrom, c = 16.077(2) Angstrom, alpha = 70.29(1)degrees, beta = 81.29(1)degrees, gamma = 84.68(1)degrees, Z = 2.