Structure, stoichiometry, reactivity. Like many monoxides, MnO adopts the rock salt structure, where cations and anions are both octahedrally coordinated.Also like many oxides, manganese(II) oxide is often nonstoichiometric: its composition can vary from MnO to MnO 1.045.. Below 118 K MnO is antiferromagnetic. MnO has the distinction of being …