A tetranuclear cyano-bridged [{Ni(HL3)}{W(CN)(8)}](2) compound in a square geometry was formed by self-assembling of {W(CN)(8)}(3-) and {NiL3}(2+) (L-3 = pentadentate ligand). The structure of the compound has been established by single crystal X-ray diffraction. The coordination sphere of the Ni ions is severely distorted with the macrocyclic ligand adopting a facial coordination with only four linkages to the metal center. The. N atom of the pendant aminopropyl arm of L 3 is no longer coordinated to the metal center but has undergone protonation during the assembling process. Magnetic measurements have revealed an unexpected antiferromagnetic behavior (J = -9 cm(-1)), which has been explained using a microscopic many-body electronic model Hamiltonian, based on DFT results. The many-body model is used to fit both the T-chi M versus T and the M versus H plots obtained from experiments.