Potassium octachlorodimolybdate

Potassium octachlorodimolybdate (systematically named potassium bis(tetrachloridomolybdate)(Mo–Mo)(4−)) is an inorganic compound with the chemical formula K
4Mo
2Cl
8 (also written as K
4[Cl
4MoMoCl
4]). 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:

2 Mo(CO)₆ + 4 HO₂CCH₃ → Mo₂(O₂CCH₃)₄ + 2 H₂ + 12 COMo₂(O₂CCH₃)₄ + 4 HCl + 4 KCl → K₄Mo₂Cl₈ + 4 HO₂CCH₃

The reaction of the acetate with HCl was first described as providing trimolybdenum compounds, but subsequent crystallographic analysis confirmed that the product contains the Mo₂Cl4–
8 ion with D_4h symmetry. The Mo–Mo distance is 2.14 Å.