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In the standard brown ring test for the nitrate ion, the brown ring complex is: $$\ce{[Fe(H2O)5(NO)]^{2+}}$$

In this compound, the nitrosyl ligand is positively charged, and iron is in a $+1$ oxidation state.

Now, iron has stable oxidation states +2 and +3. Nitrosyl, as a ligand, comes in many flavours, of which a negatively charged nitrosyl is one.

I see no reason why the iron doesn't spontaneously oxidise to +3 and reduce the $\ce{NO}$ to −1 to gain stability. But I don't know how to analyse this situation anyway. I think that there may be some nifty backbonding increasing the stability, but I'm not sure.

So, why is iron in +1 here when we can have a seemingly stable situation with iron in +3?

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    $\begingroup$ Textbooks say it is FeII and NO . FeII is rather obvious, because it is used in big excess in the reaction. And NO- is unknown to textbooks. $\endgroup$
    – Georg
    Sep 17, 2012 at 18:50
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    $\begingroup$ It's a matter of assignment. I guess it comes from stablility argument, that NO$^+$ is isoelectron to N$_2$, which is a stable triple bond ion. On the other hand, Fe$^{2+}$ or Fe$^{3+}$ will produce spin polarization around NO molecule, which may be detected from EPR spectroscopy(?) $\endgroup$
    – user26143
    Feb 11, 2014 at 0:46

8 Answers 8

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According Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of Nitric Oxide to Aquated Iron(II). An Undergraduate Text Book Reaction Revisited

The correct structure is $\ce{ [Fe^{III}(H_2O)_5(NO^{-})]^{2+} }$

For many years it was thought that iron was reduced to $\ce{Fe^{I}}$ and $\ce{NO}$ oxidized to $\ce{NO+}$, based upon an observed magnetic moment suggestive of three unpaired electrons, however, the current thinking is that high spin $\ce{Fe^{III}}$ ($S=5/2$) antiferromagnetically couples with $\ce{NO-}$ ($S=1$) for an observed spin of $S=3/2$.

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    $\begingroup$ This is a very popular question in twelfth grade in India. National level exams have asked this question with the correct answer there being that Fe is in +1 oxidation state. Feels weird to learn the wrong fact. $\endgroup$
    – Aumkaar Pranav
    Jul 16, 2020 at 4:58
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    $\begingroup$ Lots of wrong facts get learned. The whole point of science is to dig out the wrong facts on our way to the truth. $\endgroup$
    – Oscar Lanzi
    May 31, 2021 at 14:11
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Your basic assumption is incorrect: the iron in $\ce{[Fe(H2O)5NO]^{2+}}$ is Fe(III), and the ligand is $\ce{NO-}$.

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Oxidation State of "Fe" in brown ring complex depends on the binding mode of NO ligand to Iron (whether is it "bent" or "linear"). Unless we specify the IR frequencies of NO ligand for different modes, we can not say the oxidation state of "Fe". Depending on the IR stretching frequencies of NO ligand which is bound to the Iron, it can be NO+, NO- or simply NO.

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    $\begingroup$ This doesn’t really answer the question. $\endgroup$
    – Jan
    Aug 28, 2016 at 15:16
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In the brown ring complex, $\ce{[Fe(H2O)5(NO)]^{2+}}$ five water molecules are present, they are harder to stabilise the $+3$ oxidation state of $\ce{Fe}$ and hence here $\ce{NO}$ present as $-1$.
If we consider that $\ce{NO}$ is $+1$, then $\ce{Fe}$ is $+1$, which is very unstable in the co-ligand region like $\ce{H2O}$.
i.e, in brown ring complex oxidation state of Iron is $+3$.

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    $\begingroup$ Also, since another answer also gives the oxidation states, can you be specific about what you want to add to the discussion? Thanks! $\endgroup$
    – Geoff Hutchison
    Oct 21, 2014 at 15:50
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The $\ce{Fe(NO)2}$ fragment has strongly delocalized electrons, it is considered as covalently bound. The semiempirical calculations show that the electron density is polarized in a manner that allows to ascribe the delta+ charge to $\ce{NO}$ groups. Therefore formally Fe is considered +1. Remember this formal assignment is somewhat simplified... If interested, read e.g. http://pubs.rsc.org/en/content/articlehtml/2011/dt/c0dt01244k and references.

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    $\begingroup$ Welcome to Chemistry.SE! Could you summarize the contents of that article you linked. It is behind a paywall for some folks. $\endgroup$
    – Ben Norris
    Feb 10, 2014 at 16:31
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    $\begingroup$ Since the reference is about dinitrosyl iron complexes it doesn't seem relevant to the single nitrosyl complex in the question. $\endgroup$
    – DavePhD
    May 16, 2016 at 13:16
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Here iron becomes +1 due to presence of the ligand $\ce{NO}$ +1. The compound $\ce{[Fe(H2O)6]^{2+}}$ is formed when $\ce{FeSO4}$ dissolves in water during the brown ring test.

Now, when we add an aqueous solution of nitrate ions in $\ce{FeSO4}$ solution $\ce{Fe^{(II)}}$ are converted to $\ce{Fe^{(III)}}$ and $\ce{NO3-}$ are converted to $\ce{NO}$.
These $\ce{NO}$ being an odd electron compound can use three of its electrons among which it donates two electrons to $\ce{Fe^{(II)}}$ while one it shares with incompletely filled $\ce{d}$ orbital and displace one water ligand bonded with secondary valency to $\ce{Fe}$.

This way $\ce{Fe^{(II)}}$ is converted to $\ce{Fe^{(I)}}$ and not $\ce{Fe^{(III)}}$ In these regard we get the colour brown instead of yellow for $\ce{Fe^{(III)}}$ or green for $\ce{Fe^{(II)}}$

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    $\begingroup$ Are you sure? Wikipedia says it's Fe+3, see above answer. Also, I find it rather hard to follow your arguments, please be a little clearer :) $\endgroup$
    – ManishEarth
    Sep 16, 2012 at 4:56
  • $\begingroup$ @ManishEarth The wikipedia page looks absurd. It says it has Fe in +3 and NO+ which actually dont add up to the +2 charge of the complex, but to +4. $\endgroup$
    – user223679
    May 18, 2015 at 2:54
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    $\begingroup$ @ManishEarth Looks like the wikipedia page has been edited from its original by a single user which strikes controversy. And then changed again. $\endgroup$
    – user223679
    May 18, 2015 at 3:50
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The other answers are all wrong. Draw Molecular orbital diagram of $\ce{NO}$ and you will see.

That electron in antibonding orbital, it is so easy to lose, hence it easily goes to $\ce{NO^+}$ and $\ce{Fe^{(II)}}$ to $\ce{Fe^{(I)}}$.

Remember if you read something extraordinary which claims something out of daily routine (especially in chem) then it must be true.

Link : http://en.wikipedia.org/wiki/Nitrate_test

Wikipedia also says Iron is in $+1$

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The oxidation state of Fe is +1 and not +3 as most answers have stated NO+ is a strong ligand from crystal field theory(not so sure but please let me know if it is wrong ) hence Fe is in +1 Oxidation state

And as far as@Davephd answer is considered even in +1 state the spin of the compound is 3/2 as NO+ is a strong ligand

Also NO is a paramagnetic substance .so the unpaired electron it can give up to Fe to reduce it from Fe +2 to +1 and No becomes NO+

Fe+ in presence of NO+ the electonic configuration changes to 3d(2,2,1,1,1)(i dont know how to edit it so please forgive me) and so in the complex sp3d2 hybridisation takes place And so the spin still remains 3/2 here enter image description here

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  • $\begingroup$ $\ce{NO+}$ is a strong field ligand, but not a strongly binding ligand per se. It cannot bind well to cationic metals due to its cationic nature. $\endgroup$
    – Jan
    Aug 28, 2016 at 15:17
  • $\begingroup$ @Jan en.wikipedia.org/wiki/Metal_nitrosyl_complex $\endgroup$
    – MathMan
    Aug 28, 2016 at 17:34
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    $\begingroup$ and what is that supposed to tell me? All it does is reinforce my arguments as far as I skimmed it … $\endgroup$
    – Jan
    Aug 29, 2016 at 11:36

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