The first examples of metal phosphonates based on 4-(ethoxycarbonyl)naphthanen-1-yl)phosphonic acid (4-cnppH2) are reported, namely, Co(4-cnpp) (), Co2(4-cnpp)2(dptz) () and Co2(4-cnpp)2(4,4'-bpy)(H2O) () (dptz = 3,6-di(pyridin-3-yl)-1,2,4,5-tetrazine, 4,4'-bpy = 4,4'-bipyridine). The cobalt atoms are four-coordinated with distorted tetrahedral geometries in both and , while five-coordinated with a distorted trigonal bipyramidal environment in compound . Compound shows a layer structure in which the inorganic layers made up of edge-sharing {CoNO3} dimers and {PO3C} linkages are separated by the organic groups of the 4-cnpp(2-) ligands. Compound displays a different type of layer structure, where the ladder-like chains composed of corner-sharing {CoNO3} and {PO3C} tetrahedra are connected by dptz ligands with the organic groups of the 4-cnpp(2-) ligands pendent on the two sides of the layer. In , similar ladder-like chains of corner-sharing {CoNO4} and {PO3C} are also found except that the cobalt atoms within the chain are further bridged by a coordinated water molecule. The chains are cross-linked by 4,4'-bpy, leading to a three-dimensional open-framework structure. Magnetic studies reveal that dominant antiferromagnetic interactions are mediated between the Co(ii) ions in all three cases.