Hg2I2

Hg2I2, also known as mercury(I) iodide, is an inorganic compound with the chemical formula Hg2I2. It is a white crystalline solid at room temperature. Historically, it was sometimes referred to as mercury(I) subiodide or mercury(1) iodide. The compound is notable for containing the mercury(I) ion, which is a diatomic cation, [Hg-Hg]2+. This dimeric cation is responsible for some of its unique chemical properties.

Mercury(I) iodide is practically insoluble in water, but it can dissolve in solutions containing excess iodide ions due to the formation of soluble complex ions. When exposed to light, Hg2I2 undergoes decomposition, turning yellow and eventually forming mercury(II) iodide (HgI2) and metallic mercury. This photosensitivity is a characteristic property. The compound can be synthesized by reacting mercury(I) nitrate with potassium iodide. It was historically used in some medicinal preparations, though this practice is largely discontinued due to the toxicity of mercury compounds. Its use as a pigment has also been limited due to its instability and toxicity.