The electronic structure of NO2+ and NO2− has been investigated as a function of angle by means of ab initio MO LCAO SCF wavefunctions. Atomic orbitals used in the basis set are close to free-atom Hartree–Fock solutions. Several excited states, in addition to the ground states, have been obtained for the two ions. Total energies, population analyses, and one-electron orbital energies vs angle (Walsh diagrams) have been computed to aid interpretation of the wavefunctions. It turns out that the dramatic change in geometry from NO2+ (linear) to NO2− (bent, 115.4°) can be understood by the occupancy and shape of a single orbital, the 6a1 in NO2−. Our results contribute to the establishment of a firm mathematical and numerical foundation of Walsh's rules and diagrams. The present calculations also serve to substantiate and extend previous wavefunctions for these important chemical species.