Manganese(VII) oxide

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.^[1]

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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°.^[2]

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°. Re₂O₇ is not molecular but consists of crosslinked Re centers with both tetrahedral and octahedral sites.^[3]

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

Synthesis 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.^[4] Ozone is also produced, giving a strong smell to the substance.

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. "Die Kristallstruktur von Mn₂O₇" Angewandte Chemie 1987, volume 99, pages 160–161.
3. ^ Krebs, B.; Mueller, A.; Beyer, H. H. The Crystal Structure of Rhenium(VII) Oxide" Inorganic Chemistry 1969, 8, 436-443.
4. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.