Six new heterometallic cobalt(11)-lanthanide(111) complexes of formulas [Ln(bta)(H2O)(2)](2)[Co(H2O)(6)]10H(2)O [Ln = Nd(III) (1) and Eu(III) (2)] and [Ln(2)Co(bta)(2)(H2O)(8)] "6nH(2)O [Ln = Eu(III) (3), Sm(111) (4), Gd(111) (5), and Tb(III) (6)] (1-14bta = 1,2,4,5benzenetretracaboxylic acid) have been synthesized and characterized via single-crystal X-ray diffraction. 1 and 2 are isostructural compounds with a structure composed of anionic layers of [Ln(bta)(H2O)(2)] X-sandwiching mononuclear [Co(H2O)(6)] cations plus crystallization water molecules, 0 which are interlinked by electrostatic forces and hydrogen bonds, leading to a supramolecular three-dimensional network. 3-6 are also isostructural compounds, and their structure consists of neutral layers of formula [Ln(2)Co(bta)(2)(H2O)(8)] and crystallization water molecules, which are connected through hydrogen bonds to afford a supramolecular three-dimensional network. Heterometallic chains formed by the regular alternation of two nine-coordinate lanthanide(III) polyhedra [Ln(III)O-9] and one compressed cobalt(II) octahedron [Co(II)O-6] along the crystallographic c-axis are cross-linked by bta ligands within each layer of 3-6. Magnetic susceptibility measurements on polycrystalline samples for 3 6 have been carried out in the temperature range Of 2.0-300 K. The magnetic behavior of these types of Ln(III)-Co(II) complexes, which have been modeled by using matrix dagonalization techniques, reveals the lack of magnetic coupling for 3 and 4, and the occurrence of weak antiferromagnetic interactions within the Gd(III)-Gd(III) (5) and Tb(III) Tb(III) (6) dinuclear units through the exchange pathway provided by the double oxo(carboxylate) and double syn syn carboxylate bridges.