A series of heterodinuclear Cu(II)Ln(III) and Ni(II)Ln(III) complexes, [M(II)Ln(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 P2(1)2(1)2(1) 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-(HO)-O-... hydrogen bonds. Despite that, the metal centers of the neighboring units are well separated (the ranges of the shortest intermolecular contacts (&ANGS;) are ((MM)-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(II)LnIII systems has been inferred from the &UDelta;χT-M versus T plots, where &UDelta;χT-M is the difference between the values of χT-M for a Cu(II)Ln(III) system and its corresponding Ni(II)Ln(III) analogue. Ferromagnetic interactions seem to be exhibited by the (CuGdIII)-Gd-II, (CuGdIII)-Gd-II, (CuDyIII)-Dy-II, (CuHoIII)-Ho-II, (CuTmIII)-Tm-II, and (CuYbIII)-Yb-II complexes, while, for the (CuErIII)-Er-II complex, no definite conclusion could be reached. On the other hand, among the lower members of the series, the complexes of Ce-III, Nd-III, and Sm-III exhibit antiferromagnetic interactions, while the (CuPrIII)-Pr-II and (CuEuIII)-Eu-II analogues behave as spin-uncorrelated systems. The observations made here vindicate the proposition of Kahn (Inorg, Chem. 1997, 36, 930). The &UDelta;χT-M versus T plots also suggest that, for most of the Cu(II)Ln(III) complexes, the exchange interactions are fairly strong, which probably could be related to the small dihedral angle (ca. 4°) between the CuO2 and LnO(2) planes.