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 octachlorodimolybdate(II) anion, [Mo2Cl8]4−, which features a quadruple Mo–Mo bond

The compound is prepared in two steps from molybdenum hexacarbonyl:

2 Mo(CO)6 + 4 CH3CO2H → (CH3CO2)4Mo2 + 2 H2 + 12 CO

(CH3CO2)4Mo2 + 4 HCl + 4 KCl → K4[Mo2Cl8] + 4 CH3CO2H

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≣MoCl4]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, 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.