Elemental Potassium as both an oxidizer and a reducer
Compounds having potassium anions are referred to as potassides and are rare, usually involving K- within a cyrpt such as:



Crystalline Salts of Na- and K- (Alkalides) that Are Stable at Room Temperature J. Am. Chem. Soc. vol. 121, pages 10666-10667 describes preparation of K+(aza222)K- , where "aza222" is the compound in the above figure, but with the 6 H's replaced with methyl groups.  The compound is formed by dissolving potassium metal in methyl amine solution of the methylated aza222.