A series of heterodinuclear CuIILnIII and Ni IILnIII complexes, [MIIL1Ln III(NO3)3] (M = Cu or Ni; Ln = Ce-Yb), with the hexadentate Schiff base compartmental ligand N,N′-ethylenebis(3- ethoxysalicylaldiimine) (H2L1) have been synthesized and characterized. The X-ray crystal structure determinations of 13 of these compounds reveal that they are all isostructural. All of these complexes crystallize with the same orthorhombic P212121 space group with closely similar unit cell parameters. Typically, the structure consists of a diphenoxo-bridged 3d-4f dinuclear core, self-assembled to two dimensions due to the intermolecular nitrate⋯copper(II) or nitrate⋯nickel(II) semicoordination and weak C-H⋯O hydrogen bonds. Despite that, the metal centers of the neighboring units are well separated (the ranges of the shortest intermolecular contacts (Å) are (M⋯M) 7.46-7.60, (Ln⋯Ln) 8.56-8.69, and (M⋯Ln) 6.12-6.20). Variable-temperature (5-300 K) magnetic susceptibility measurements of all the complexes have been made. The nature of exchange interactions in the Cu IILnIII systems has been inferred from the ΔχMT versus T plots, where ΔχMT the difference between the values of χMT for a CuIILn III system and its corresponding NiIILn IIIanalogue. Ferromagnetic interactions seem to be exhibited by the CuIIGdIII, CuIITbIII, Cu IIDyIII, CuIIHoIII, Cu IITmIII, and CuIIYbIII complexes, while, for the CuIIErIII complex, no definite conclusion could be reached. On the other hand, among the lower members of the series, the complexes of CeIII, NdIII, and SmIII exhibit antiferromagnetic interactions, while the CuIIPrIII and CuIIEuIII analogues behave as spin-uncorrelated systems. The observations made here vindicate the proposition of Kahn (Inorg. Chem. 1997, 36, 930). The ΔχMT versus T plots also suggest that, for most of the CuIILnIII complexes, the exchange interactions are fairly strong, which probably could be related to the small dihedral angle (ca. 4°) between the CuO2 and LnO2 planes. © 2005 American Chemical Society.