Linear (ONiO) and triangular (Ni(O-2)) isomers of NiO2 are investigated by multiconfiguration self-consistent field (MCSCF) and multireference configuration interaction (MRCI) calculations. For ONiO, the ground electronic term is a (1)Sigma(+)(g) term. The lower-lying excited terms are (3)Pi(g), (1)Pi(g), and (5)Pi(u) at relative energies of 0.55, 0.95, and 1.20 eV, respectively. For Ni(O-2), the ground electronic term is a (1)A(1) term with an energy of 1.53 eV with respect to the ONiO ground state. Lower-lying excited terms are B-5(2), (5)A(1), and B-3(2) at 0.58, 0.62, and 0.73 eV with respect to the (1)A(1) state, respectively. A transition structure between the ground states of both isomers has been located with an energy of 2.76 eV above the ONiO ground state. For the fragmentation Ni(O-2) -> Ni + O-2 the (electronic) reaction energy is estimated to 1.15 eV. The wave function based results demonstrate the failure of previous density functional investigations and emphasize the importance of a multireference treatment.