Synthesis, crystal structures, and magnetic properties of two types of carboxylate-bridged Cu(II)2Ln(2)(III) complexes, formulated as [Cu(2)Ln(2)(bet)(10)(H2O)(8)](ClO4)(10). 2H(2)O (1; Ln(III) = La-III, Ce-III, or Gd-III; bet = betaine) and [Cu(2)Ln(2)-(bet) (12)(ClO4)(2)](ClO4)(8) (2; Ln(III) - Gd-III or Sm-III), have been described, 1 . La features a carboxylate-bridged, tetranuclear [Cu2La2(bet)(10)(H2O)(8)](10+) cation, crystallizing in the triclinic space group P (1) over bar, with a = 12.778(4) Angstrom, b = 15.553(5) Angstrom, c = 16.041(5) Angstrom, alpha = 110.18(2)degrees, beta = 100.72(2)degrees, gamma = 105.03(2)degrees, and Z = 1; 1 . Ce is isomorphous with 1 . La, crystallizing in the triclinic space group P (1) over bar, with a 12.730(2) Angstrom, b = 15.489(2) Angstrom, c = 15.987(3) Angstrom, alpha = 110.28(1)degrees, beta = 100.70(1)degrees, gamma = 105.00(2)degrees, and Z = 1. In both 1 . La and 1 . Ce, a Cu-II atom is quadruply bridged to a Ln(III) atom by the mu(2)-carboxylate groups into a dinuclear subunit, and a pair of such dinuclear subunits is bridged by two carboxylate groups to form a tetranuclear cation. In the tetranuclear cation, each Cu-II atom is coordinated in a square pyramid by four carboxy oxygen atoms at the basal positions and by one aqua ligand at the apical position whereas each Ln(III) atom is coordinated in a monocapped square antiprism by six carboxy oxygen atoms and three aqua ligands. 1 . Gd has been characterized to be an analogue of 1 . La. 2 . Gd consists of a different tetranuclear [Cu2Gd2(bet)(12)(ClO4)(2)](8+) cation, crystallizing in the orthorhombic space group Pmcb with a = 12.323(3) Angstrom, b = 17.119(4) Angstrom, c = 26.381(7) Angstrom, and Z = 2; 2 . Sm is isomorphous with 2 . Gd, crystallizing in the orthorhombic space group Pmcb, with a = 12.496(2) Angstrom, b = 17.378(3) Angstrom, c = 26.767(5) Angstrom, and Z = 2. In both 2 . Gd and 2 . Sm, a pair of dinuclear subunits analogous to those in 1 . La are linked by a quadruple carboxylate bridge between the pair of Ln(III) atoms into a tetranuclear cation, where the apical ligand of the square pyramid about each Cu-II atom is a perchlorate and each Ln(III) atom is coordinated in a square antiprism by eight carboxy oxygen atoms. The magnetic behavior of these complexes obeys the Curie-Weiss law, showing very weak antiferromagnetic interaction in the solid. Therefore, it may be concluded that shielding of the 4f electrons by the outer shell electrons very effectively precludes significant coupling interactions between the lanthanoid 4f electrons and copper 3d electrons in a carboxylate-bridged system.