The synthesis and physical characterization of a series of lanthanide (Ln(III)) and nickel (Ni-II) mixed trimetallic complexes with the heptadentate (N4O3) amine phenol ligand H(3)trn [tris(2'-hydroxybenzylaminoethyl)amine] has been accomplished in order to extend our understanding of how amine phenol ligands can be used to coaggregate d- and f-block metal ions and to investigate further the magnetic interaction between these ions. The one-pot reaction in methanol of stoichiometric amounts of H(3)trn with NiX2.6H(2)O (X = ClO4, NO3) followed by addition of the corresponding LnX(3).6H(2)O salt, and then base, produces complexes of the general formula [LnNi(2)(trn)(2)]X.nH(2)O.The complexes were characterized by a variety of analytical techniques. Crystals of five of the complexes were grown from methanol solutions and their structures were determined by X-ray analysis: [PrNi2(trn)(2)(CH3OH)]ClO4.4CH(3)-OH.H2O, [SmNi2(trn)(2)(CH3OH)]NO3.4CH(3)OH.2H(2)O, [TbNi2(trn)(2)(CH3OH)]NO3.4CH(3)OH.3H(2)O, [ErNi2(trn)(2)(CH3OH)]-NO3.6CH(3)OH, and [LuNi2(trn)(2)(CH3OH)]NO3.4.5CH(3)OH.1.5H(2)O. The [LnNi(2)(trn)(2)(CH3OH)](+) complex cation consists of two octahedral Ni-II ions, each of which is encapsulated by the ligand trn(3-) in an N4O2 coordination sphere with one phenolate O atom not bound to Ni-II. Each [Ni(trn)](-) unit acts as a tridentate ligand toward the Ln(III) ion via two bridging and one nonbridging phenolate donors. Remarkably, in all of the structurally characterized complexes, Ln(III) is seven-coordinate and has a flattened pentagonal bipyramidal geometry. Such uniform coordination behavior along the whole lanthanide series is rare and can perhaps be attributed to a mismatch between the geometric requirements of the bridging and nonbridging phenolate donors. Magnetic studies indicate that ferromagnetic exchange occurs in the Ni-II/Ln(II) complexes where Ln = Gd, Tb, Dy, Ho, or Er.