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What is the structure of $\ce{FeSO4 \cdot NO}$ that is formed when $\ce{NO}$ is passed through ferrous sulphate solution? If it is brown ring complex then why does the complex sphere break on heating to give $\ce{NO}$? In general, when do the complex spheres break ?

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  • $\begingroup$ There has been some previous discussion on this topic here, though I don't think it qualifies as a duplicate. $\endgroup$ – Nicolau Saker Neto May 13 '14 at 11:48
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What is the structure of $\ce{FeSO4 \cdot NO}$ that is formed when $\ce{NO}$ is passed through ferrous sulphate solution?

The structure is octahedral.

The Fe ion is at the center of the octahedron.

Five water molecules and the NO molecule occupy the vertices of the octahedron.

Sulfate is a separate spectator ion.

The overall charge of the iron coordination complex is 2+.

If it is brown ring complex, why does the complex sphere break on heating to give $\ce{NO}$? In general, when do the complex spheres break ?

"Brown ring" refers to the macroscopic appearance of the product in a tube. This is unrelated to the chemical structure. The binding of NO to Fe is an exothermic reaction. In accordance with the Van't Hoff Equation, the equillibrium constant shifts in favor of the reverse reaction as temperature increases.


For detailed infomation see Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of Nitric Oxide to Aquated Iron(II)

Interestingly, for many years it was thought the iron was reduced to Fe(I), the NO being oxidized to NO+; however, according to the above reference, the current thinking is that the iron is oxidized to Fe(III), and the NO is reduced to NO-.

In other words, the complex is $\ce{ [Fe^{III}(H_2O)_5(NO^{-})]^{2+} }$

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    $\begingroup$ Dave is absolutely correct. Indeed, Wieghardt unambiguously established oxidation state of iron and nitrosyl (non-innocent/redox-active ligand) in brown ring complex using EPR & Mössbaue spectral data. $\endgroup$ – user25214 Jan 29 '16 at 16:59

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