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Topic: Why U(VI) exists as Uranyl  (Read 12685 times)

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Offline pranay29

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Why U(VI) exists as Uranyl
« on: January 10, 2012, 09:00:14 AM »
Why U(VI) exists as uranyl ion i.e. UO2+2, not as U+6?

Offline Mitch

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Re: Why U(VI) exists as Uranyl
« Reply #1 on: January 10, 2012, 12:37:33 PM »
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.
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Offline gippgig

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Re: Why U(VI) exists as Uranyl
« Reply #2 on: January 10, 2012, 09:34:21 PM »
So why does heptavalent Np & Pu exist as the XO53- complex?

Offline Mitch

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Re: Why U(VI) exists as Uranyl
« Reply #3 on: January 11, 2012, 12:12:58 AM »
So why does heptavalent Np & Pu exist as the XO53- complex?

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

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Re: Why U(VI) exists as Uranyl
« Reply #4 on: January 11, 2012, 01:34:25 PM »
I thought that U6+ existed, for example UF6 (Uranium Hexafluoride)
http://en.wikipedia.org/wiki/File:Uranium_hexafluoride_crystals_sealed_in_an_ampoule.jpg


Offline Borek

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Re: Why U(VI) exists as Uranyl
« Reply #5 on: January 11, 2012, 02:02:59 PM »
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.
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Offline Mitch

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Re: Why U(VI) exists as Uranyl
« Reply #6 on: January 11, 2012, 02:12:39 PM »
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+).
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Offline snorkack

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Re: Why U(VI) exists as Uranyl
« Reply #7 on: February 13, 2014, 05:07:37 PM »
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.

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