The synthesis, X-ray crystal structure and magnetic properties of two oxalato-bridged dinuclear Cu-II complexes, [L-Cu(mu-ox)Cu-L](ClO4)(2), where ox = oxalate and L = 1,4,7-trimethyl-1,4,7-triazacyclononane (Me(3)tacn) or 1-benzyl-4,7dimethyl-1,4,7-triazacyclononane (BzMe(2)tacn) are reported, as well as the X-ray crystal structure of the mononuclear complex, [Cu(Me(3)tacn)(OH2)(2)](ClO4)(2), which features an extended H-bonding network between the ligand and the perchlorate counterions. The crystal structures of the dinuclear complexes both feature a centrosymmetric complex cation in which the two copper centres exist in a distorted square-pyramidal geometry, with each end capped by a facially coordinated tacn unit, and are bridged by an oxalate anion. Variable-temperature magnetic susceptibility measurements revealed strong antiferromagnetic interactions between the Cull centres, with 2J values of ca. -300 cm(-1), even though the Cu...Cu separation is very long, viz. 5.18 angstrom (BzMe(2)tacn) and 5.47 angstrom (Me(3)tacn). The separation for the Me(3)tacn complex is the longest separation reported for this type of Cull complex. No pattern could be detected between the J values and structural features for 21 oxalato-bridged complexes. (c) Wiley-VCH Verlag GmbH Co.