Three new isomorphous complexes [Ni(o-van-en)LnCl3(H2O)] [H2(o-van-en) = N,N'-ethylene-bis(3-methoxysalicylaldiminate; Ln = Ce (1), Gd (2), Dy (3)] were prepared by a stepwise reaction using mild conditions and were structurally characterised as dinuclear molecules in which Ni and Ln are coordinated by the compartmental Schiff base ligand (o-van-en)= and doubly bridged by O atoms. While the nickel(ii) centre is diamagnetic within the N2O2 square-planar coordination of the Schiff base ligand, the lanthanide atoms are octa-coordinated to give an {LnCl3O5} chromophore with a fac-arrangement of the chlorido ligands. AC magnetic measurements revealed that all three complexes, including the nominally isotropic Gd(iii) system, show field induced slow magnetic relaxation with two or three relaxation channels: at T = 1.9 K the low-frequency relaxation time is τLF(1) = 0.060 s at BDC = 0.5 T, τLF(2) = 0.37 s at BDC = 0.3 T, and τLF(3) = 1.29 s at BDC = 0.15 T.