Eighteen trinuclear Ni(2)(II)Ln(III) complexes of 2,6-di(acetoacetyl)pyridine (H2L) (Ln = La-Lu except for Pm) were prepared by a "one-pot reaction" of H2L, Ni(NO3)(2)center dot 6H(2)O, and Ln(NO3)(3)center dot nH(2)O in methanol. X-ray crystallographic studies indicate that two L2- ligands sandwich two Ni-II ions with the terminal 1,3-diketonate sites and one Ln(III) ion with the central 2,6-diacylpyridine site, forming the trinuclear [Ni(2)Ln(L)(2)] core of a linear NiLnNi structure. The terminal Ni assumes a six-coordinate geometry together with methanol or water molecules, and the central Ln assumes a 10-coordinate geometry together with two or three nitrate ions. The [Ni(2)Ln(L)(2)] core is essentially coplanar for large Ln ions (La, Ce, Pr, Nd) but shows a distortion with respect to the two L2- ligands for smaller Ln ions. Magnetic studies for the Ni(2)Ln complexes of diamagnetic La-III and Lu-III indicate an antiferromagnetic interaction between the terminal Ni-II ions. A magnetic analysis of the Ni2Gd complex based on the isotropic Heisenberg model indicates a ferromagnetic interaction between the adjacent Ni-II and Gd-III ions and an antiferromagnetic interaction between the terminal Ni-II ions. The magnetic properties of other Ni(2)Ln complexes were studied on the basis of a numerical approach with the Ni2La complex and analogous Zn(2)Ln complexes, and they indicated that the Ni-II-Ln(III) interaction is weakly antiferromagnetic for Ln = Ce, Pr, and Nd and ferromagnetic for Ln = Gd, Tb, Dy, Ho, and Er.