Thank you AWK.
So NO replaces one of the water ligands in Fe2+ (aq) and form a brown complex ion.
Am I correct to think that it only occur at the conc sulfuric acid end because the H+ conecntration is high enough to shift the equilibrium to favour the formation of NO?
and does the concentrated acid stablise the brown complex ion? it seemed that it is destoryed when diluted with water or is it just diluted out?
This answer is close to it, but not precisely so.
NO indeed replaces a water ligand, but by doing so, it is oxidized to a nitrosyl ligand, NO+
. So, you get the compound [Fe(H2O)5
, but in this compound there is no NO ligand, but a NO+
ligand. The iron has oxidation state +1 in this complex, which is quite remarkable! Normally, iron has oxidation state +2 or +3.
On dilution, the nitrosyl complex is destroyed. It hydrolyses. The NO+
entity is not stable in water and is quickly converted to NO2-
. Iron in oxidation state +1 is not stable in water and so, the nitrite is reduced to gaseous NO.