A series of tetranuclear copper(II) and nickel(II) complexes is described, all of which form by a strict self-assembly process involving just a single ligand and the metal salt. The ligands POAP, POAPZ, PZOAP, and 6POAP contain terminal pyridine and pyrazine residues bound to a central flexible diazine subunit (N-N) and contain two potentially bridging groups (alkoxo and diazine). in all cases but one the metals are linked by alkoxo oxygens alone, forming essentially square [M-4(mu(2)-O)(4)] clusters. A rectangular copper(II) complex [Cu-4(mu(2)-N-2)(2)-(mu(2)-O)(2)] involves a mixture of two alkoxo and two diazine bridges. The square Cu4O4 clusters exhibit ferromagnetic coupling between the metal centers, associated with the orthogonal arrangement of magnetic orbitals, while for the Ni4O4 clusters the nickel(II) centers are coupled antiferromagnetically. The [Cu-4(mu(2)-N-2)(2)(mu(2)-O)(2)] cluster exhibits strong antiferromagnetic coupling through a trans Cu(N-N)Cu bridging arrangement, typical for systems of this sort. X-ray structures are reported for [Cu-4 (POAP-H)(4)(H2O)(2)](NO3)(4).4H(2)O (2), [Cu-4(POAPZ-H)(4)(H2O)](NO3)(4). 3H(2)O (3), [Cu-4(6POAP-H)(4)](ClO4)(4) (4), [Cu-4(PZOAP-H)(4)](NO3)(4).3H(2)O (5), [Ni-4(POAP-H)(4) (H2O)(4)](NO3)(4).8H(2)O (6), and [Ni-4(PZOAP-H)(4)(H2O)4](ClO)(4))(4).5H(2)O (9). 2 crystallized in the monoclinic system, space group C2/c, with a = 20.479(3) Angstrom, b = 14.920(2) Angstrom, c = 19.671(3) Angstrom, beta = 90.591(4)degrees, and Z = 8.3 crystallized in the monoclinic system, space group P2(1)/c, with a 14.12(1) Angstrom, b = 14.116(3) Angstrom, c = 29.013(4) Angstrom, beta = 93.50(3)degrees, and Z = 4.4 crystallized in the monoclinic system, space group C2/c, with a = 22.646(4) Angstrom, b = 25.847,(5) Angstrom, c 12.349(5) Angstrom, beta = 110.34(2)degrees, and Z = 4.5 crystallized in the monoclinic system, space group P2/n, with a = 14.3573(4) Angstrom, b = 10.8910(6) Angstrom, c 20.5360(10) Angstrom, beta = 96.975(4)degrees, and Z = 4.6 crystallized in the triclinic system, space group P (1) over bar, with a = 12.0509(6) Angstrom, b = 12.7498(6) Angstrom, c = 23.1208 Angstrom, alpha = 93.1050(10)degrees, beta = 100.1500(10)degrees, gamma = 108.5050(11)degrees, and Z = 2.9 crystallized in the orthorhombic system, space group Pbcn, with a = 14.368(4) Angstrom, b = 25.369(7) Angstrom, c = 18.479(5) Angstrom, and Z = 4.