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MgO2 is Calaverite structured and crystallizes in the monoclinic C2/m space group. The structure is two-dimensional and consists of one MgO2 sheet oriented in the (0, 0, 1) direction. Mg is bonded to six equivalent O atoms to form distorted edge-sharing MgO6 octahedra. There are four shorter (2.07 Å) and two longer (2.16 Å) Mg–O bond lengths. O is bonded in a trigonal non-coplanar geometry to three equivalent Mg atoms.
MgO2 is Pyrite-like structured and crystallizes in the cubic Pa-3 space group. The structure is three-dimensional. Mg is bonded to six equivalent O atoms to form MgO6 octahedra that share corners with twelve equivalent MgO6 octahedra and corners with six equivalent OMg3O tetrahedra. The corner-sharing octahedral tilt angles are 69°. All Mg–O bond lengths are 2.11 Å. O is bonded to three equivalent Mg and one O atom to form OMg3O tetrahedra that share corners with three equivalent MgO6 octahedra and corners with fifteen equivalent OMg3O tetrahedra. The corner-sharing octahedral tilt angles are 72°. The O–O bond length is 1.51 Å.
MgO2 is pyrite-like structured and crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Mg is bonded to six equivalent O atoms to form edge-sharing MgO6 octahedra. All Mg–O bond lengths are 2.11 Å. O is bonded in a 4-coordinate geometry to three equivalent Mg and one O atom. The O–O bond length is 1.49 Å.
Three competing bonding mechanisms have been identified for the alkaline-earth metal superoxides: these result in a change in the optimal structure and ground state as the alkaline-earth metal becomes heavier. For example, BeO2 has a linear 3Sigma(-)g ground-state structure, whereas both CaO2 and SrO2 have C(2v)1A1 structures. For MgO2, the theoretical calculations are less definitive, as the 3A2 C(2v) structure is computed to lie only about 3 kcal/mol above the 3Sigma(-)g linear structure. The bond dissociation energies for the alkaline-earth metal superoxides have been computed using extensive Gaussian basis sets and treating electron correlation at the modified coupled-pair functional or coupled-cluster singles and doubles level with a perturbational estimate of the triple excitations.