Unusually, this anion occurs in the “non-classical” di•iodides and di•hydrides formed by some lanthanides, such as lanthanum and cerium. A sodium salt of this anion is only stable in helium at extremely high pressure. Like permanganate, preparing salts of this anion often requires that the cation be enclosed by a crown ether or a cryptand. In solution, this anion imparts a gold or copper color at a high concentration, while a low concentration is instead dark (*) blue. Solutions of this anion are often made by dissolving an alkali metal in ammonia, allowing for this anion to serve as the reducing agent in the Birch reduction. (10[1])A difference in the potentials of two metals causes the flow of this anion (10[1])between two half-cells. For 10 points, galvanic cells cause the flow of what negatively charged subatomic particles? ■END■