Two novel dinuclear-based polynuclear and tetranuclear Cu(II) compounds with mixed HxPO4(3-x)- bridges (x = 1, 2), [Cu-2(bpy)(2)(mu,eta(2)-HPO4)mu,eta(1)-H2PO4)(mu,eta(2)-H2PO4)](n) (1) and [Cu-4(phen)(4)(mu(3),eta(2) -HPO4)(2)(mu,eta(2) -H2PO4)(2)(H2PO4)(2)](H2O)(4) (2) (in which bpy = 2,2'-bipyridine and phen = 1, 10-phenanthroline) have been synthesized and characterized structurally and magnetically. Both Cu(II) ions in a dimeric unit in 1 are in a distorted square pyramidal environment and linked by mu,eta(2)-HPO4 and mu,eta(1) -H2PO4 ligands. Complex 1 is a dinuclear-based polymeric Cu(II) derivative of [Cu-2(bpy)(2)(mu,eta(2) -HPO4)(mu,eta(1)-H2PO4)(mu,eta(2) -H2PO4)] units linked by mu,eta(2)- H2PO4 ligand. Consequently, the Cu(II) ions are alternatively doubly-bridged and singly-bridged along the polymeric chain. In case of 2, each Cu(II) ion displays a distorted square-pyramidal geometry, with two mu(3),eta(2) -HPO4 groups and two mu,eta(2)-H2PO4 groups bridging, and two non-bridging H2PO4 groups, involving four copper atoms in a rather unique variety of coordination modes. The magnetic susceptibility measurements reveal a weak antiferromagnetic interaction for both compounds. Magneto-structural trends are discussed, along with their EPR and electronic reflectance spectra and compared with those of related complexes. (C) 2006 Elsevier Ltd. All rights reserved.