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 (15[1])anion imparts a gold or copper color at a high concentration, while a low concentration is instead dark (*) blue. (15[1])Solutions of (10[1])this anion (10[1])are (10[1])often made by dissolving an alkali metal in ammonia, (10[2])allowing (-5[1])for this anion to (10[1])serve as the reducing agent in the Birch (-5[1])reduction. A (10[1])difference in the (10[1])potentials of two (10[1])metals causes the flow (10[1])of this anion (-5[1])between two (10[2])half-cells. For 10 points, galvanic cells (10[1])cause the flow (10[1])of what negatively charged subatomic particles? (10[3])■END■ (10[2])