[pranay29]

Why U(VI) exists as uranyl ion i.e. UO₂⁺², not as U⁺⁶?

[Mitch]

I'm going to give a watered down answer. Not many metals (I can't think of one) can exist as a pure +6. Think about it, it would be so positive that it would tend to scrape electrons from anything near it due to Coulomb attraction.

[gippgig]

So why does heptavalent Np & Pu exist as the XO₅^3- complex?

[Mitch]

So why does heptavalent Np & Pu exist as the XO₅^3- complex?

Unfortunately, I neither have a watered down answer nor an advance answer to address that question.

[vmelkon]

I thought that U⁶⁺ existed, for example UF₆ (Uranium Hexafluoride)
http://en.wikipedia.org/wiki/File:Uranium_hexafluoride_crystals_sealed_in_an_ampoule.jpg

[Borek]

I thought that U⁶⁺ existed, for example UF₆ (Uranium Hexafluoride)

Not in a water solution. I have a gut feeling UF₆ is more covalent than ionic.

[Mitch]

I thought that U⁶⁺ existed, for example UF₆ (Uranium Hexafluoride)

Good point, I stand corrected. However, since the subllimation point is so low (56.5 °C) it would be better to consider it a covalent molecule and not as an ionic one. How many traditional ionic salts can melt at such a low temperature?

Although, the oxidation state for uranium is (VI), I think we have to be very careful in extrapolating this to uranium behaving as a cation (U⁶⁺).

[snorkack]

So why does heptavalent Np & Pu exist as the XO₅^3- complex?

Probably something related to ligand field preferred geometry. Heptavalent elements occur as XO₄^- (X=Cl, Br, I, Mn, Tc, Re), XO₆^5- (X=I), I think I also forms some IO₅^3- complexes.  Not sure why Np and Pu prefer their specific geometry.