What compound is formed in this case?
Iron(II) reduces nitrite to nitrogen monoxide which forms a brown pentaaquanitrosyliron(II) complex with the iron(II) ions.

$$\ce{NO2^{-} + Fe^{2+} + 2H+ -&gt; NO + Fe^{3+} + H2O}$$

$$\ce{[Fe(H2O)6]^{2+} + NO -&gt; [Fe(H2O)5NO]^{2+} + H2O}$$

Iron(II) can also reduce nitrate to nitrogen monoxide. This is used in the brown ring test to determine the presence of nitrate in aqueous solutions.