Potassium octachlorodimolybdate

Potassium octachlorodimolybdate(4−)

Identifiers
CAS Number	25448-39-9
Properties
Chemical formula	H4Cl8K4Mo2
Molar mass	631.9 g/mol
Appearance	red crystals
Density	2.54 g/cm^3
Solubility in water	soluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

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

2 Mo(CO)₆ + 4 HO₂CCH₃ → Mo₂(O₂CCH₃)₄ + 2 H₂ + 12 CO

Mo₂(O₂CCH₃)₄ + 4 HCl + 4 KCl → K₄Mo₂Cl₈ + 4 HO₂CCH₃

The reaction of the acetate with HCl was first described as providing trimolybdenum compounds,^[3] 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.^[4]

References

1. Brignole, A. B.; Cotton, F. A.; Dori, Z. (1972). "Rhenium and Molybdenum Compounds Containing Quadruple Bonds". Inorg. Synth. Inorganic Syntheses. 13: 81–89. doi:10.1002/9780470132449.ch15. ISBN 978-0-470-13244-9.
2. Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books. ISBN 0-935702-48-2.
3. Allison, G. B.; Anderson, I. R.; Sheldon, J. C. (1967). "The Preparation of Halogenotrimolybdate(II) Compounds". Aus. 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.