Manganese heptoxide

Manganese(VII) oxide
IUPAC name Manganese(VII) oxide
Other names Permanganic acid Manganic oxide dimanganese heptoxide
Identifiers
CAS number	12057-92-0
Jmol-3D images	Image 1
SMILES O=(O=)(O=)MnOMn(=O)(=O)=O
Properties
Molecular 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/cm3
Melting point	5.9 °C, 279.1 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
Y heptoxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Manganese(VII) oxide 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]

Properties

The crystalline form of this chemical compound is dark green. 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^{[clarification needed]}. 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 sulfuric acid to form its anhydride, Mn₂O₇.

2 KMnO₄ +(cold) 2 H₂SO₄ → Mn₂O₇ + H₂O + 2 KHSO₄

Mn₂O₇ can react further with sulfuric acid to give the remarkable cation MnO₃⁺, which is isoelectronic with CrO₃:^{[citation needed]}

Mn₂O₇ + 2 H₂SO₄ → 2 [MnO₃]⁺[HSO₄]⁻ + H₂O

Mn₂O₇ decomposes near room temperature, explosively so at > 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.^{[citation needed]}

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.