The synthesis and properties of a novel hetero-tetranuclear: compound [Cr-2(bpy)(4)(mu(2)-O)(4)Nb-2(C2O4)(4)]center dot 3H(2)O (1; bpy = 2,2'-bipyridine), investigated by single-crystal X-ray diffraction, magnetization measurements, IR, UV/visibl'e spectroscopy, electron paramagnetic resonance (EPR; X- and Qbands and high-field), and density functional thebry (DFT) calcUlations, are reported. Crystal structure of 1 (orthorhombic Pcab spce group) consists of a square-shaped macrocyclic {Cr-2(mu-O)(4)Nb-2} core in which Cr-III and Nb-V ions are alternately bridged by Oxo ions and three uncoordinated water molecules. The intramolecular diStances through the,-O-Nb-v-O-bridges are 7.410(2) and 7.419(2) angstrom, while diagonal separation is 5.406(2) angstrom. The temperature dependence of magnetization M(T) evidences an anti-ferromagnetic ground state, which Originates from a magnetic interaction between two Cr-III ions of spin 3/2 through two triatomic-O-Nb-v-O- diamagnetic bridges. A spin Hamiltonian appropriate for polynuclear isolated magnetic units was used. The best-fitting curve for this model is :obtained with the parameters g(cr) = 1.992(3), J = -12.77(5) cm(-1), and vertical bar D vertical bar = 0.17(4) cm(-1). The Cr-III center dot center dot center dot Cr-III dimer model is confirmed by EPR spectra, which exhibit a pronounced change of their shape around the temperature corresponding to the intradimer coupling J. The EPR spectra simulations and DFT calculations reveal the presence of a Single-ion anisotropy that is close to being uniaxial, D = -0.31 cm(-1) and E = 0.024 cm(-1).