Manganese(VII) oxide

Chembox new
Name = Manganese(VII) oxide
ImageFile = Mn2O7.png ImageName = Manganese(VII) oxide
ImageFile1 = Manganese-heptoxide-3D-balls.png ImageName1 = Ball-and-stick model of manganese heptoxide
IUPACName = Manganese(VII) oxide
OtherNames = Manganic oxide
dimanganese heptoxide
Section1 = Chembox Identifiers
SMILES = O=(O=)(O=)MnOMn(=O)(=O)=O
CASNo = 12057-92-0
RTECS =
Section2 = Chembox Properties
Formula = Mn₂O₇
MolarMass = 221.87 g/mol
Appearance = dark red oil (room temp.)
Density = 2.79 g/cm³
Solubility = dec.
MeltingPt = 5.9 °C
BoilingPt = explodes on heating
sublimes at −10 °C
Section3 = Chembox Structure
Coordination = bitetrahedral
CrystalStruct = monoclinic
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = explosive, strong oxidizer
RPhrases =
SPhrases =
Section8 = Chembox Related
OtherCpds = Re₂O₇
KMnO₄
Tc₂O₇

Manganese(VII) oxide is the chemical compound with the formula Mn₂O₇. This volatile liquid is highly reactive and more often discussed than observed or intentionally prepared. It is a dangerous oxidizer that was first described in 1860. [Aschoff, H. Ann. Phys. Chem. Ser. 2 volume 111 (1860) page 217 and page 224.]

Key properties

The crystalline compound is red. It is soluble in CCl₄, 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°. [Simon, A.; Dronskowski, R.; Krebs, B.; Hettich, B. "Die Kristallstruktur von Mn₂O₇" Angewandte Chemie 1987, volume 99, pages 160–161.]

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, [Krebs, B.; Mueller, A.; Beyer, H. H. The Crystal Structure of Rhenium(VII) Oxide" Inorganic Chemistry 1969, 8, 436-443.] in the vapour phase it is molecular with a similar structure to Tc₂O₇. [ Wells A.F. (1962) "Structural Inorganic Chemistry" 3d edition Oxford University Press ]

One distinction of Mn₂O₇ is that it is charge-neutral. "Molecular" oxides that are neutral or cationic are often oxidants.

ynthesis and reactions

Mn₂O₇ arises as a dark red oil by the addition of H₂SO₄ to KMnO₄. The reaction initially produces permanganic acid, HMnO₄ (also described with the formula HOMnO₃). Permanganic acid spontaneously loses water to form its anhydride, Mn₂O₇.:2 KMnO₄ + 2 H₂SO₄ → Mn₂O₇ + H₂O + 2 KHSO₄

Mn₂O₇ can react further with sulfuric acid to give the remarkable cation MnO₃⁺, which is isoelectronic with CrO₃::Mn₂O₇ + 2 H₂SO₄ → 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. [Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.] Ozone is also produced, giving a strong smell to the substance.

References