Two cobalt phosphonates, [Co2 (2,2'-bpy)2 (H2 O)(pbtcH)] (1) and [Co2 (H2 O)(pbtcH)(phen)2 ] (2; pbtcH5 =5-phosphonatophenyl-1,2,4-tricarboxylic acid, 2,2'-bpy=2,2'-bipyridine, phen=1,10-phenanthroline), with layer structures are reported. Compound 1 contains O-C-O and O-P-O bridged tetramers of Co4 , which are further connected by pbtcH(4-) units to form a layer. In compound 2, the cobalt tetramers made up of water-bridged Co2 dimers and O-P-O linkages are connected into a layer by pbtcH(4-) units. Upon dehydration, compounds 1 and 2 experience single-crystal-to-single-crystal (SC-SC) structural transformations to form [Co2 (2,2'-bpy)2 (pbtcH)] (1 a) and [Co2 (pbtcH)(phen)2 ] (2 a), respectively. The process is reversible in each case. Notably, a breathing effect is observed for 1, accompanied by pore opening and closing due to the reorientation of the coordinated 2,2'-bpy molecules. The transformation was also monitored by in situ IR measurements. Magnetic studies reveal that antiferromagnetic interactions are mediated between the magnetic centers in compounds 1 and 1 a, whereas ferromagnetic interactions are dominant in compound 2.