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Iron (VIII)


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#1 ferrocene

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Posted 11 December 2006 - 10:39 AM

Does anyone have information about the chemistry of iron in oxidation state VIII ? (The chemistry of iron (VI) is well documented)

Mellorís Treatise on Inorganic Chemistry devotes only a couple of paragraphs to the preparation of potassium perferrate K2FeO5 and iron tetraoxide FeO4. I remember as a teenager trying out the experiments described, but didnít obtain the same results.

I have googled extensively but found nothing of help.

(How do I convert a number to sub-script?)
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#2 woelen

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Posted 11 December 2006 - 11:51 AM

I have the book "Chemistry of the Elements" and this books says that no compounds of iron(VIII) have been isolated and confirmed by independent groups. So, any compounds like FeO4 or K2FeO5 are very doubtful. I think that no iron(VIII) compounds exist, and certainly not at room temperature.

==================================================

For layout: Use [ ce]...[ /ce] sections:

[ ce]K_{2}FeO_{4}[ /ce] ---> [LaTeX Error: Syntax error]
[ ce]CrO_{4}^{2-}[ /ce] ---> [LaTeX Error: Syntax error]

You need to skip the spaces after the [ of course.

See also here: http://www.sciencefo...ead.php?t=10188
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#3 ferrocene

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Posted 11 December 2006 - 12:42 PM

Thanks for this.

Who is the book's author and when was it written?

If my memory serves me correctly, Mellor stated that K2FeO5 was obtained by heating Fe2O3, KOH and a large excess of KNO3. A green melt is obtained, which becomes a green solid on cooling. When I tried it, I certainly obtained a green melt, but it became white when cooled. Sadly, I can't remember anything else.

I had doubts then about the existence of iron (VIII). If it doesn't exist, then I wonder why not?
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#4 RyanJ

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Posted 11 December 2006 - 05:05 PM

Who is the book's author and when was it written?


Look here under "useful books" :)

I had doubts then about the existence of iron (VIII). If it doesn't exist, then I wonder why not?


I'd agree with Woelen, doing some quick research turned up no useful information about iron(VIII) compounds so I'd guess their either can't exist (due to stability problems) or only exist at low energies and then only in solution.
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#5 woelen

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Posted 12 December 2006 - 08:09 AM

Why doesn't it exist? That is an interesting question. There definitely is a pattern among all transition metals.

All first row transition metals are most stable in lower oxidation states, while the second and third row transition metals are more stable in their higher oxidation states.

E.g. V - Nb - Ta. Vanadium has extensive aqueous chemistry for +2, +3, +4 and +5 oxidation state, +5 is actually a fairly strong oxidizer. Nb only has fleeting existance in the +4 oxidation state in aqueous chemistry, +5 is by far the most stable oxidation state, and Ta is more or less limited to the +5 oxidation state, and only under very specific anhydrous conditions it exists in lower oxidation states.

The effect is more marked with Cr - Mo - W. Cr is strongly oxidizing in the +6 oxidation state, while for Mo and W this is the most stable oxidation state.

With Mn - Tc - Re this effect is even stronger. Permanganate is one of the strongest common oxidizers, while pertechnate and perrhenate are not oxidizing. I do have some ammonium perrhenate, and that chemical is rather inert. Only strong reductors, like sodium borohydride and metallic zinc in acid (nascent hydrogen) are sufficiently strong reductors to reduce the ReO4(-) ion to a lower oxidation state.

The triad Fe - Ru - Os is the turn-over point. Fe(VIII) would be so strongly oxidizing, that it cannot exist anymore in normal environments. Molecules or ions with this would be so unstable that they fall apart through internal oxidation/reduction. Even iron(VI) already is very strongly oxidizing. On the other hand, RuO4 is only a moderately strong oxidizer and the same is true for OsO4. Ru(VI) is quite stable in alkaline environment, Ru(VII) is quite stable in acidic environments.

For the further transition metals, a similar thing is true, but less pronounced. Usually the first row allows oxidation states +2 (and sometimes +3) and the higher rows then allow +3 and sometimes +4. Platinum also allows +6, but that is an extremely strong oxidizer, e.g. PtF6 is capable of oxidizing oxygen.

I think that really understanding this pattern requires quantum mechanics. What I did is just describing observations, QM computations, however, reveal these properties.
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#6 ferrocene

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Posted 12 December 2006 - 11:15 AM

Thanks for this, Woelen - interesting points you raise, quite a few of them new to me. The evidence certainly suggests that Fe (VIII) does not exist. However, I remember reading (quite some time ago) why KBrO4 and the perbromates could not possibly exist - but that didn't stop them from existing. I also noted a detailed description of OsF8 - and subsequently a report saying it definitely does not exist.

I am curious then as to what might be happening when Fe2O3, KOH and a large excess of KNO3 are heated. (Unfortunately, I didn't copy the equation that Mellor gave - but this was many years ago) I remember clearly that a dark green melt was produced, that it turned white when it cooled and solidified (contrary to what Mellor said). I cannot think of any iron compound which would behave in such a way under those conditions. There are a number of iron compounds which are white, but I can't think of any which would fit this particular bill.

Unfortunately, I'm not in a position to try this experiment again.
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#7 SergMih2015

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Posted 1 February 2015 - 07:54 AM

On this issue, there is some information in the article "Yu. M. Kiselev, Yu. D. Tretiyakov. The problem of oxidation state stabilisation and some regularities of a Periodic system of the elements " on page 371. 

https://yadi.sk/i/Q0hVmJj4eNuyw.


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#8 gh2o

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Posted 9 December 2016 - 09:49 PM

Thanks for this, Woelen - interesting points you raise, quite a few of them new to me. The evidence certainly suggests that Fe (VIII) does not exist. However, I remember reading (quite some time ago) why KBrO4 and the perbromates could not possibly exist - but that didn't stop them from existing. I also noted a detailed description of OsF8 - and subsequently a report saying it definitely does not exist.

I am curious then as to what might be happening when Fe2O3, KOH and a large excess of KNO3 are heated. (Unfortunately, I didn't copy the equation that Mellor gave - but this was many years ago) I remember clearly that a dark green melt was produced, that it turned white when it cooled and solidified (contrary to what Mellor said). I cannot think of any iron compound which would behave in such a way under those conditions. There are a number of iron compounds which are white, but I can't think of any which would fit this particular bill.

Unfortunately, I'm not in a position to try this experiment again.

 

I hope you are still around. I too am VERY interested in this topic - FeO4 or K2FeO5. I have (many years ago) experience with synthesizing perbromate using F2 gas per E. Appleton's method for my masters research project. I did a one-time attempt to make FeO4 using the same method starting with K2FeO4 (ferrate VI) in concentrated KOH. After fluorinating the purple solution for a few minutes, it turned white, fizzed and bubbled for a few minutes, then slowly turned a yellowish-brown. Don't know if I made it, then it decomposed in the caustic solution. Like you have never be in a position to try this again or attempt any other procedure. Please respond if you are around. Thanks.


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