So why does heptavalent Np & Pu exist as the XO53- complex?
Probably something related to ligand field preferred geometry. Heptavalent elements occur as XO4-
(X=Cl, Br, I, Mn, Tc, Re), XO65-
(X=I), I think I also forms some IO53-
complexes. Not sure why Np and Pu prefer their specific geometry.