In the nitrate test iron(II) sulfate is added to a solution suspected of containing nitrates followed by slowly adding sulfuric acid so that it forms a layer below the aqueous layer. If nitrates are out there they reveal themselves by a visible brown ring between the two layers. The chemical reactions taking place are oxidation of Fe(II) to Fe(III) and formation of gaseous nitric oxide (NO) , nitric oxide is then reduced further to the NO- ion in (Fe(H2O)5(NO))SO4. This ancient chemical test is back in the news (briefly) thanks to Georg Monsch and Peter Klüfers who in Angewandte have been re-investigating what the brown stuff actually is. They are not the first in this investigation but others before them failed because NO gas escapes very easily and X-ray grade crystals have been impossible to grow. In their 2019 effort Monsch and Klüfers have been more successful because they swapped the sulphate counter ion for a much larger gallium pinacolato one. To do this they mixed iron triflate, gallium triflate, perfluoropinacol , sodium hydroxide and nitric oxide.
Crystallography for this metal nitrosyl complex has iron sitting in a octahedral_molecular_geometry with 5 water units and the NO ligand not linear (Fe-N-O making an angle). In a bent M-NO unit the bond order in NO is 2 rather than 3 and pi backbonding is considered less relevant. The compound is described as FeNO7 in Enemark-Feltham notation having 6 (iron d-electrons) + 1 = 7 electrons of pi symmetry and again in any metal nitrosyl complex above a value of 6 a bent bond is expected.
And what about the oxidation state? Earlier researchers have described the compound as either Fe+(NO+), Fe2+NO0 or Fe3+(NO)-, that is, nobody really knows. NO+ is isoelectronic with carbon monoxide as in a metal carbonyl. NO- is associated with bent bonds. Basically Monsch and Klüfers throw their hands up in the air. Computations seem to favor a neutral nitrosyl oxide. Mössbauer spectroscopy is ordinarily very good at detecting oxidation state differences but arguments are presented that it is of no use for this particular compound. The definition of an oxidation state is also fuzzy. The key quote: "an instructor may decide to abstain from teaching oxidation states just on the basis of this compound, or, likewise, to demonstrate either the limits of a formal concept or to discuss sensible future extensions of the oxidation state idea - the latter with the aim to include the current computational progress"