Chelated heteroatoms in polyoxometalates and the topological equivalence of [CoIII(en)] to type II cis-dioxometal centers. Synthesis and structure of [[Co(en)(mu-OH)2Co(en)}[PW10O37Co(en)]2]8- and [K subset[Co(en)WO4][WO(H2O)](PW9O34)2]12-.