Profiting from a multicoordinating nitronyl nitroxide radical, i.e. a functionalized nitronyl nitroxide biradical ligand, a family of novel tetranuclear lanthanide complexes, formulated as [RE4(hfac)(12)(NITPhO-3Pybis)(2)] (RE = Gd 1, Tb 2, Dy 3, Ho 4, and Y 5; NITPhO-3Pybis = 5-(3-pyridinyloxy)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene; hfac = hexafluoroace-tylacetonate) were successfully constructed and characterized. In these complexes, the designed functionalized nitronyl nitroxide biradical ligand functions as the chelating and/or bridging ligand to bind the lanthanide ions, resulting in tetranuclear octa-spin lanthanide complexes with a circle-shaped arrangement. Direct-current magnetic data show that antiferromagnetic interaction dominates in the Gd derivative, while ferromagnetic coupling plays a leading role in complex Y, in which the relevant magnetic exchange parameters were obtained through applicable magnetic models. Dynamic magnetic studies of Tb and Dy analogues exhibit apparent frequency-dependent out-ofphase signals, which are typical features for slow magnetic relaxation behavior. The values of U-eff and tau(0) were obtained as follows: U-eff = 10.5 K and tau(0) = 6.6 x 10(-7 )s for the Tb complex and U-eff = 5.2 K and tau(0) = 2.5 X 10(-6) s for the Dy compound. Intriguingly, the butterfly shaped hysteresis loop is found for the Tb analogue. Guided by fluorescence spectra, the representative peaks are identified for the Tb derivative.