Three polynuclear complexes containing copper(II) and pyrazino[2,3-f ][4,7]phenanthroline (pap) as the basic building blocks have been prepared [Cu2(pap)(H2O)2(NO3)3]n[NO3]n1, [Cu4(pap)4Cl7]nCln·15nH2O 2, and [Cu4(pap)4(H2O)4(C4O4)2]n[C4O4]n[NO3]2n·12nH2O 3, and their crystal structures and variable-temperature magnetic susceptibilities determined. Compound 1 is a single stranded zigzag chain where pap and nitrate alternate as bridges between the copper atoms. The copper coordination geometry is to a first approximation distorted square pyramidal, but with an additional semi-coordinated oxygen from non-bridging nitrate groups. The bridging nitrate coordinates in the apical position to both copper atoms. In 2 the basic structural unit is a cyclic, tetranuclear entity where copper atoms are bridged by pap. Two different pap bridges are present, one through only equatorial positions, the other coordinating equatorially and axially. These tetranuclear units are linked, through diagonally opposite corners, into chains by single atom chloro-bridges. The copper coordination geometry is distorted elongated octahedral. In 3 a tetranuclear building block, with structural characteristics similar to those found in 2, is present, but in this case coordinated squarate (3,4-dihydroxycyclobut-3-en-1,2-dionate) links these units into sheets. Neighbouring sheets are firmly connected by hydrogen bonds through uncoordinated squarate and water into a three-dimensional supramolecular structure featuring channels running normal to the sheets. The Cu ⋯ Cu separations across bridging pap are 6.809 Å in 1, 6.887 and 7.297 Å in 2, and 6.936 and 7.250 Å in 3. Variable-temperature susceptibility measurements on 1–3 reveal the occurrence of very weak intramolecular antiferromagnetic interactions between copper(II) ions through bridging pap (the largest value of J being −2.5 cm−1).