The synthesis and structures of fifteen new cobalt complexes containing phosphonate ligands are reported. The compounds also utilize carboxylates and 6-chloro-2-pyridonate (chp) as co-ligands. The majority of the compounds are decametallic: [Co(10)(chp)(12)(O(3)PPh)(2)(O(2)CPh)(4)(H(2)O)(4)], [Co(10)(chp)(12)(O(3)PPh)(2)(O(2)C(t)Bu)(4)(H(2)O)(4)], [Co(10)(chp)(12)(O(3)PPh)(2)(O(2)CPh(t)Bu)(4)(H(2)O)(4)], [Co(10)(chp)(6)(O(3)PCH(2)Ph)(2)(O(2)CPh)(8)(F)(2) (H(2)O)(2)(EtOAc)(2)], [Co(10)(chp)(8)(O(3)PCH(2)Ph)(2)(O(2)CPh)(8)(F)(2)(MeCN)(2)](HNEt(3))(2), [Co(10)(chp)(6)(O(3)PCH(2)Ph)(2)(O(2)C(t)Bu)(8)(F)(2)(H(2)O)(2)(MeCOMe)(2)], [Co(10)(chp)(6)(O(3)PMe)(2) (O(2)C(t)Bu)(8)(F)(2)(MeCN)(4)], [Co(10)(chp)(6)(O(3)PEt)(2)(O(2)CPh)(8)(F)(2)(MeCN)(4)], [Co(10)(chp)(6)(O(3)POct)(2)(O(2)CPh)(8)(F)(2)(MeCN)(4)], [Co(10)(chp)(8)(Hchp)(2)(O(3)PCH(2)Nap) (O(2)CPh)(7)(OH)(3)(H(2)O)], [Co(10)(chp)(12)(O(3)PPh)(2)(O(2)CPh-2-Ph)(4)(H(2)O)(4)] and [Co(10)(chp)(12)(O(3)PMe)(2)(O(2)CPh-2-Ph)(4)(H(2)O)(4)]. Two nine-metal cages and one hexametallic cage are also reported: [Co(9)(chp)(9)(O(3)P(t)Bu)(O(2)C(t)Bu)(6)(OH)], [Co(9)(chp)(7)(O(3)PCH(2)Ph)(2)(O(3)PCH(2)Ph)(O(2)CCPh(3))(5)(OH)(H(2)O)(2)(MeCN)] and [Co(6)(chp)(6)(Hchp)(2)(O(3)P(t)Bu)(O(2)CPh-2-Ph)(3)(F)(H(2)O)](HNEt(3))(Cl). Magnetic studies show predominantly anti-ferromagnetic exchange interactions between the cobalt(ii) sites, with diamagnetic ground states for most of the compounds studied.