Polyanions complete ligands

Polyoxometallates. Polyoxometallates of the group 6 transition metals (iso- and hetero-polyoxoanions) form a special class of metallate ligands for the actinide elements. These species can incorporate other atoms as either primary or secondary (peripheral) heteroatoms. Primary heteroatoms are necessary to complete the polyoxoanion structure secondary heteroatoms can be removed without disruption of the stable polyanion unit. The early actinides serve in both roles in known compounds. 

---