Molecular and Electronic Structures of M₂O₇ (M = Mn, Tc, Re)

The molecular and electronic structures of the group 7 heptoxides were investigated by computational methods as both isolated molecules and in the solid-state. The metal-oxygen-metal bending angle of the single molecule increased with increasing atomic number, with Re₂O₇ preferring a linear structure. Natural bond orbital and localized orbital bonding analyses indicate that there is a three-center covalent bond between the metal atoms and the bridging oxygen, and the increasing ionic character of the bonds favors larger bond angles. The calculations accurately reproduce the experimental crystal structures within a few percent. Analysis of the band structures and density of states shows similar bonding for all of the solid-state heptoxides, including the presence of the three-center covalent bond. DFT+U simulations show that PBE-D3 underpredicts the band gap by ∼0.2 eV due to an undercorrelation of the metal d conducting states. Homologue and compression studies show that Re₂O₇ adopts a polymeric structure because the Re-oxide tetrahedra are easily distorted by packing stresses to form additional three-center covalent bonds.