Manganese heptoxide

Manganese(VII) oxide

Names
IUPAC name Manganese(VII) oxide
Other names Manganic oxide dimanganese heptoxide
Identifiers
CAS Number	12057-92-0
EC Number	235-025-8
Jmol interactive 3D	Image
PubChem	13879826
SMILES O=[Mn](=O)(=O)O[Mn](=O)(=O)=O
Properties
Chemical formula	Mn2O7
Molar mass	221.87 g/mol
Appearance	dark red oil (room temp.), green if in contact with sulfuric acid
Density	2.79 g/cm^3
Melting point	5.9 °C (42.6 °F; 279.0 K)
Boiling point	explodes on heating sublimes at −10 °C
Solubility in water	decomposes to permanganic acid, HMnO4
Structure
Crystal structure	monoclinic
Coordination geometry	bitetrahedral
Hazards
Main hazards	explosive, strong oxidizer, very corrosive
Related compounds
Related compounds	Re2O7 KMnO4 Tc2O7
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

Manganese(VII) oxide (manganese heptoxide) is an inorganic compound with the formula Mn₂O₇. This volatile liquid is highly reactive and more often discussed than intentionally prepared. It is a dangerous oxidizer and was first described in 1860.^[1] It is the acid anhydride of permanganic acid.

Properties

The crystalline form of this chemical compound is dark green. The liquid is green by reflected light and red by transmitted light. It is soluble in carbon tetrachloride, and decomposes when in contact with water. It melts at only 5.9 °C, and sublimes at −10 °C. These properties indicate a nonpolar molecular species, which is confirmed by its structure. The molecules consist of a pair of tetrahedra that share a common vertex. The vertices are occupied by oxygen atoms and at the centers of the tetrahedra are the Mn(VII) centers. The connectivity is indicated by the formula O₃Mn-O-MnO₃. The terminal Mn−O distances are 1.585 Å and the bridging oxygen is 1.77 Å distant from the two Mn atoms. The Mn−O−Mn angle is 120.7°.^[2]

It contains manganese in its highest oxidation state. This oxidation state, +7, is shared by permanganates, which are more stable compounds.

Pyrosulfate, pyrophosphate, and dichromate adopt structures similar to that of Mn₂O₇. Probably the most similar main group species is Cl₂O₇. Focusing on comparisons within the transition metal series, Tc₂O₇ and Mn₂O₇ are structurally similar but the Tc−O−Tc angle is 180°. Solid Re₂O₇ is not molecular but consists of crosslinked Re centers with both tetrahedral and octahedral sites;^[3] in the vapour phase it is molecular with a similar structure to Tc₂O₇.^[4]

Synthesis and reactions

Mn₂O₇ arises as a dark green oil by the addition of concentrated H₂SO₄ to KMnO₄. The reaction initially produces permanganic acid, HMnO₄ (structurally, HOMnO₃), which is dehydrated by cold sulfuric acid to form its anhydride, Mn₂O₇.

2 KMnO₄ + 2 H₂SO₄ → Mn₂O₇ + H₂O + 2 KHSO₄

Mn₂O₇ can react further with sulfuric acid to give the remarkable manganyl(VII) cation MnO+
3, which is isoelectronic with CrO₃:

Mn₂O₇ + 2 H₂SO₄ → 2 [MnO
3]+
[HSO
4]−
+ H₂O

Mn₂O₇ decomposes near room temperature, explosively so above 55 °C. The explosion can be initiated by striking the sample or by its exposure to oxidizable organic compounds. The products are MnO₂ and O₂.^[5] Ozone is also produced, giving a strong smell to the substance. The ozone can spontaneously ignite a piece of paper impregnated with an alcohol solution. The production of manganese heptoxide is an undesirable byproduct in many situations.

References

1. Aschoff, H. Ann. Phys. Chem. Ser. 2 volume 111 (1860) page 217 and page 224.
2. Simon, A.; Dronskowski, R.; Krebs, B.; Hettich, B. (1987). "The Crystal Structure of Mn₂O₇". Angew. Chem. Int. Ed. Engl. 26: 139–140. doi:10.1002/anie.198701391.
3. Krebs, B.; Mueller, A.; Beyer, H. H. (1969). "The Crystal Structure of Rhenium(VII) Oxide". Inorganic Chemistry 8: 436–443. doi:10.1021/ic50073a006.
4. Wells A.F. (1962) Structural Inorganic Chemistry 3d edition Oxford University Press
5. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.