O2, similarly to N2, has the ability to bind either side-on or end-on.
In the solid state, MgO2 has a cubic pyrite-type crystal structure with 6-coordinate Mg2+ ions and O22− peroxide-groups, according to experimental data [REF] and evolutionary crystal structure prediction,[REF] the latter predicting a phase transition at the pressure of 53 GPa to a tetragonal structure with 8-coordinate Mg2+ ions.