New homo- and heterobimetallic tetranuclear complexes of formula Cu(4)(mpba)(Me(4)en)(4)(H(2)O)(4)(4).3H(2)O (1), Cu(4)(mpba)(Me(4)en)(4)(H(2)O)(4)(4).2H(2)O (2), Cu(4)(ppba)(Me(4)en)(4)(H(2)O)(4)(4).2H(2)O (3), Cu(4)(mpba)(dipn)(4)(4).3H(2)O (4), Cu(4)(ppba)(dipn)(4)(4).2H(2)O (5), and Cu(2)Ni(2)(ppba)(dipn)(4)(H(2)O)(2)(4) (6) [mpba = N,N'-1,3-phenylenebis(oxamate), ppba = N,N'-1,4-phenylenebis(oxamate), Me(4)en = N,N,N',N'-tetramethylethylenediamine, and dipn = dipropylenetriamine] have been synthesized and structurally and magnetically characterized. Complexes 1-6 have been prepared following a molecular-programmed self-assembly method, where a heteropolytopic tetranucleating phenylenedioxamato bridging ligand (L = mpba or ppba) is bound to four metal ions of identical or different natures (M = Cu(II) and/or Ni(II)) with partially blocked coordination sites by bi- or tridentate polyamine terminal ligands (L' = Me(4)en or dipn). The structures of 1-6 consist of cationic tetranuclear Cu(II)(2)M(II)(2) entities with an overall (4)R rack-type architecture, which is made up of two oxamato-bridged homo- (1-5) or heterodinuclear (6) Cu(II)M(II) units (M = Cu and Ni) connected through either a meta- (1, 2, and 4) or a para-substituted (3, 5, and 6) phenylene spacer between the Cu(II) ions. The magnetic properties of 1-6 have been interpreted according to their "dimer-of-dimers" structure [H = -J(S(1).S(2) + S(3).S(4)) - J'(S(1).S(3)) with S(1) = S(3) = S(Cu) = 1/2 and S(2) = S(4) = S(M) = 1/2 (M = Cu) or 1 (M = Ni)]. The homometallic Cu(II)(4) complexes exhibit either strong (-J = 330-350 cm(-1)) or weak-to-moderate (-J = 4.8-87.1 cm(-1)) antiferromagnetic intradimer couplings through the oxamato bridge, depending on the bi- (1-3) or tridentate (4 and 5) nature of the terminal ligand, respectively. The heterometallic Cu(II)(2)Ni(II)(2) complex with a tridentate terminal ligand (6) shows instead a moderate antiferromagnetic intradimer coupling (-J = 50 cm(-1)). Otherwise, the nature and magnitude of the interdimer coupling cannot be unambiguously determined except for the pair of homo- and heterometallic Cu(II)(2)M(II)(2) complexes [M = Cu (5) and Ni (6)] with the p-phenylenedioxamato bridging ligand and a tridentate terminal ligand, which show a weak antiferromagnetic interdimer coupling (-J' = 14 and 23 cm(-1)) across the para-substituted phenylene spacer.