The reaction of squarate (C 4 O 4 2-, dianion of 3,4-dihydroxycyclobut-3-ene-1,2-dione) and croconate (C 5 O 5 2-, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione) with the dinuclear [Cu 2 (apox)] 2+ copper(II) complex [H 2 apox = N,N'bis(3-aminopropyl)oxamide] in aqueous solution affords the compounds of formula [Cu 2 (apox)(C 4 O 4 )(H 2 O) 2 ] n •n H 2 O (1) and [Cu 4 (apox) 2 (C 5 O 5 ) 2 ]•6 H 2 O (2). Crystals of 1 are monoclinic, space group C2/c, with a = 12.5527(9), b = 7.4161(6), c = 18.5198(12) Å, β = 100.578(6)°and Z = 4. Crystals of 2 are triclinic, space group P1 fl with a = 9.732(5), b = 9.795(2), c = 10.285(3) Å, α = 84.95(2), β = 77.30(3), γ = 85.61( 3)°and Z = 1. The structure of complex 1 consists of apox-bridged copper(II) dinuclear entities that are linked through bis-monodentate squarato ligands yielding neutral chains where copper atoms are bridged alternately by apox and squarato groups. The apox and squarato ligands exhibit bis-terdentate and µ-1,3-bridging modes. Each copper atom exhibits a distorted square pyramidal environment: the equatorial plane comprises the oxygen and nitrogen atoms of the amide, the nitrogen atom of the amine and an oxygen atom from the squarato, whereas the apical position is occupied by a water molecule. The intrachain copper-copper separations across oxamidato and squarato are 5.272(1) and 7.981(1) Å, respectively. The structure of 2 comprises centrosymmetric tetranuclear [Cu 4 (apox) 2 (C 5 O 5 ) 2 ] molecules, which are linked through weak axial Cu-O(croconato) bonds into sheets. Within the tetranuclear unit the copper atoms are alternately bridged by the bis-terdentate apox ligand and croconato, which coordinates asymmetrically bis-bidentate. The two crystallographically independent copper atoms exhibit tetragonally elongated octahedral surroundings with terdentate apox and one croconate oxygen in the equatorial plane and two croconate oxygens in the axial positions. The copper-copper separations within the tetranuclear unit are 5.203(2) and 4.601(2) Å across apox and croconato, respectively. Variable-temperature magnetic susceptibility measurements show the occurrence of a very strong antiferromagnetic interaction (J = -338 (1) and -350 cm -1 (2)) across the bridging apox. These values lie within the range of those reported for parent apox-bridged copper(II) complexes where the apox ligand exhibits the trans conformation. © 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS copper(II) complexes / crystal structures / magnetic properties / alternating chains / sheetlike polymers Version française abrégée -Squarate et croconate pour le design de complexes mono-et bidimensionnels de Cu(II) à pont oxamidato : synthèse, structures cristallines et propriétés magnétiques de [Cu 2 (apox)(C 4 O 4 )(H 2 O) 2 ] n •n H 2 O and [Cu 4 (apox) 2 (C 5 O 5 ) 2 ]•6 H 2 O. Les composés [Cu 2 (apox)(C 4 O 4 )(H 2 O) 2 ] n •n H 2 O (1) et [Cu 4 (apox) 2 (C 5 O 5 ) 2 ]•6 H 2 O (2) ont été obtenus par réaction des ligands squarate (C 4 O 4 2-, dianion du 3,4-dihydroxy-* Correspondence and reprints.