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 an 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]

References

1. Brignole, A. B.; Cotton, F. A.; Dori, Z. (1972). "Rhenium and Molybdenum Compounds Containing Quadruple Bonds". Inorganic Syntheses. Vol. 13. pp. 81–89. doi:10.1002/9780470132449.ch15. ISBN 978-0-470-13244-9. {{cite book}}: |journal= ignored (help)
2. Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books. ISBN 978-0-935702-48-4.
3. Allison, G. B.; Anderson, I. R.; Sheldon, J. C. (1967). "The Preparation of Halogenotrimolybdate(II) Compounds". Aust. J. Chem. 20 (5): 869–876. doi:10.1071/CH9670869.
4. Brencic, Jurij V.; Cotton, F. Albert (1969). "Octachlorodimolybdate(II) Ion. Species with a Quadruple Metal–Metal Bond". Inorg. Chem. 8: 7–10. doi:10.1021/ic50071a002.