Question

The oxidation state of Fe in [Fe(H₂O)₅(NO)]SO₄
a) +1
b) +2
c) +4
d) +3

Answer 1

In the standard brown ring test for the nitrate ion, the brown ring complex is:

[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 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?