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.

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°.

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, in the vapour phase it is molecular with a similar structure to Tc2O7.

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  &rarr;  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  &rarr;  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. Ozone is also produced, giving a strong smell to the substance.