Mercury(I) nitrate is an inorganic compound, a salt of mercury and nitric acid with the formula Hg2(NO3)2. A yellow solid, the compound is used as a precursor to other Hg22+ complexes. The structure of the hydrate has been determined by X-ray crystallography. It consists of a [H2O-Hg-Hg-OH2]2+ center, with a Hg-Hg distance of 254 pm.[2] Reactions Mercury(I) nitrate is formed when elemental mercury is combined with dilute nitric acid (concentrated nitric acid will yield mercury(II) nitrate). Mercury(I) nitrate is a reducing agent which is oxidized upon contact with air. Mercuric (II) nitrate reacts with elemental mercury (0) to form mercurous (I) nitrate (comproportionation reaction):[citation needed] Hg(NO3)2 + Hg ⇌ Hg2(NO3)2 Solutions of mercury(I) nitrate are acidic due to slow reaction with water: Hg2(NO3)2 + H2O ⇌ Hg2(NO3)(OH) + HNO3 Hg2(NO3)(OH) forms a yellow precipitate. If the solution is boiled, or exposed to light, mercury(I) nitrate undergoes a disproportionation reaction yielding elemental mercury and mercury(II) nitrate:[3] Hg2(NO3)2 ⇌ Hg + Hg(NO3)2 These reactions are reversible; the nitric acid formed can redissolve the basic salt.[citation needed]