Manganese(II) oxide

Like many metal monoxides, MnO adopts the rock salt structure, where cations and anions are both octahedrally coordinated. Also like many metal oxides, manganese(II) oxide is often nonstoichiometric: its composition can vary from MnO to MnO_1.045.^[3]

Manganese(II) oxide undergoes the chemical reactions typical of an ionic oxide. Upon treatment with acids, it converts to the corresponding manganese(II) salt.^[3] Oxidation of manganese(II) oxide gives manganese(III) oxide.

MnO occurs in nature as the rare mineral manganosite.
It is prepared commercially by reduction of MnO₂ with hydrogen, carbon monoxide, or methane, e.g.:^[2]

MnO₂ + H₂ → MnO + H₂O

MnO₂ + CO → MnO + CO₂

Upon heating to 450 °C, manganese(II) nitrate gives a mixture of oxides, denoted MnO_2−x, which can be reduced to the monoxide with hydrogen at ≥750 °C.^[4] MnO is particularly stable and resists further reduction.^[5] MnO can also be prepared by heating the carbonate:^[6]

MnCO₃ → MnO + CO₂

This calcining process is conducted anaerobically, lest Mn₂O₃ form.

An alternative route, most interest for demonstration purposes, is the "oxalate method". Also applicable to the synthesis of ferrous oxide and stannous oxide, it entails heating in an oxygen-free atmosphere (often CO₂), hydrated manganese(II) oxalate:^[7]

MnC₂O₄·2H₂O → MnO + CO₂ + CO + 2 H₂O

Together with manganese sulfate, MnO is a component of fertilizers and food additives. Many thousands of tons are consumed annually for this purpose. Other uses include: a catalyst in the manufacture of allyl alcohol, pigmenting ceramics and paints, coloring glass, bleaching tallow, and textile printing.^[2]

Below 118 K, MnO is antiferromagnetic.^[3] MnO has the distinction of being one of the first compounds^[8] to have its magnetic structure determined by neutron diffraction, the report appearing in 1951.^[9] This study showed that the Mn²⁺ ions form a face centered cubic magnetic sub-lattice where there are ferromagnetically coupled sheets that are anti-parallel with adjacent sheets.

Similar to NiO, MnO is classed as a strongly correlated material because of the localised 3d states associated with Mn atoms, and is an electrical insulator.^[10][11] Conventional approximations to the exchange-correlation functional in density functional theory (DFT) such as the local spin-density approximation (LSDA) severely underestimate the band gap of the material, and can even predict it to be metallic, depending on the choice of magnetic configuration.^[10][12] However, improved descriptions of the material's electronic structure, such as hybrid exchange-correlation functionals, DFT+U^[13], the GW approximation^[12], self-interaction corrected DFT^[14], or coupled cluster theory^[15], all recover the band gap with significantly improved accuracy. It has also been shown that such improved methods predict a band gap regardless of the choice of magnetic state.^[16]