The reactions of the dinuclear copper complexes [Cu-2(L)(OAc)] [H3L = N,N'-(2-hydroxypropane-1,3-diyl)bis-(salicylaldimine) or [Cu-2(L')(OAc)] (H3L' = N,N'-(2-hydroxypropane-1,3-diyl)bis(4,5-dimethylsalicylaldimine)] with various phosphonic acids, RPO3H2 (R = t-Bu, Ph, c-C5H9, c-C6H11 or 2,4,6-i-Pr-3-C6H2), leads to the replacement of the acetate bridge affording tetranuclear copper(II) phosphonates, [Cu-4(L)(2)(t-BuPO3)](CH3OH)(2)(C6H6) (1), [Cu-4(L)(2)-(PhPO3)(H2O)(2)(NMe2CHO)](H2O)(2) (2), [Cu-4(L')(2)(C5H9PO3)](CH3OH)(2) (3), [Cu-4(L')(2)(C6H11PO3)](MeOH)(4)(H2O)(2) (4) and [Cu-4(L')(2)(C30H46P2O5)](PhCH3) (5). The molecular structures of 1-4 reveal that a [RPO3](2-) ligand is involved in holding the four copper atoms together by a 4.211 coordination mode. In 5, an in situ formed [(RPO2)(2)O](4-) ligand bridges two pairs of the dinuclear subunits. Magnetic studies on these complexes reveal that the phosphonate ligand is an effective conduit for magnetic interaction among the four copper centers present; a predominantly antiferromagnetic interaction is observed at low temperatures.