Potassium octachlorodimolybdate (systematically named potassium bis(tetrachloromolybdate)(Mo–Mo)(4−)) is an inorganic compound with the chemical formula K4[Mo2Cl8]. It is known as a red-coloured, microcrystalline solid. The anion is of historic interest as one of the earliest illustrations of a quadruple bonding. The salt is usually obtained as the pink-coloured dihydrate. The compound is prepared in two steps from molybdenum hexacarbonyl: The reaction of the acetate with HCl was first described as providing trimolybdenum compounds, but subsequent crystallographic analysis confirmed that the salt contains the [Cl4Mo\qMoCl4](4–) anion, with D4h symmetry, in which the two Mo atoms are linked by a quadruple bond. Each Mo atom is bounded with four Cl− ligands by a single bond. Each MoCl4 group is a [regular square pyramid](https://bliptext.com/articles/[square](https://bliptext.com/articles/square)-[pyramid](https://bliptext.com/articles/pyramid-geometry)al-molecular-geometry), with an Mo atom at the apex, and four Cl atoms at the vertices of the square base of the pyramid. The Mo–Mo distance is 214 pm.