Potassium octachlorodimolybdate

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

Potassium octachlorodimolybdate(4−)

Identifiers
CAS Number	25448-39-9 Y
3D model (JSmol)	Interactive image
CompTox Dashboard (EPA)	DTXSID00330354 DTXSID501045175, DTXSID00330354
InChI InChI=1S/8ClH.4K.2Mo/h8*1H;;;;;;/q;;;;;;;;4*+1;2*+2/p-8 Key: IPKVUDXOBCRDJX-UHFFFAOYSA-F
SMILES [K+].[K+].[K+].[K+].Cl[Mo-2](Cl)(Cl)(Cl)$[Mo-2](Cl)(Cl)(Cl)Cl
Properties
Chemical formula	K4[Mo2Cl8]
Molar mass	631.89 g·mol−1
Appearance	red crystals
Density	2.54 g/cm3
Solubility in water	soluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

The compound is prepared in two steps from molybdenum hexacarbonyl:^[1][2]

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,^[3] but subsequent crystallographic analysis confirmed that the salt contains the [Cl₄Mo≣MoCl₄]⁴⁻ anion, with D_4h 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.^[4]