The synthesis of a new Ni-II-Y-III binuclear complex with a marked elongation axis in the first coordination sphere of the Ni-II ion is presented. Its zero-field splitting (ZFS) is studied by means of magnetic data and state-of-the-art ab initio calculations. A good agreement between the experimental and theoretical ZFS parameter values is encountered, validating the whole approach. The magnetic anisotropy axes are extracted from the ab initio calculations, showing that the elongation axis around the Ni-II ion corresponds to the hard axis of magnetization and that the sign of the axial D parameter is imposed by this axis. The Ni-Y axis is found to be an easy axis of magnetization, which is, however, not significant according to the sign of D. The already reported [(H2O)Ni(ovan)(2)(mu-NO3)Y(ovan)(NO3)]center dot H2O (ovan = o-vanillin) complex is then revisited. In this case, the elongation axis in the Ni-II coordination sphere is less marked and the ZFS is dominated by the effect of the Y-III ion belonging to the second coordination sphere. As a consequence, the D parameter is negative and the low-temperature behavior is dominated by the Ni-Y easy axis of magnetization. A competition between the first coordination sphere of the Ni-II ion and the electrostatic effect of the Y-III ion belonging to the second coordination sphere is then evidenced in both complexes, and the positive and negative D parameters are then linked to the relative importance of both effects in each complex.