Laser-ablated niobium and tantalum atoms react with O-2 to give MO, MO2, MO2-, and MO2+ products, which are identified from oxygen isotopic substitution, on their matrix infrared spectra and from DFT calculations of isotopic frequencies. Annealing allows diffusion and further reaction to form (O-2)MO2 complexes, which are characterized by four fundamentals, On the basis of the nu(3) vibrational frequencies for the (MO2)-O-16 and (MO2)-O-18 isotopic molecules, the metal dioxide valence angles are estimated to be 108 +/- 5 degrees for NbO2 and 106 +/- 5 degrees for TaO2; the molecular cations have the smaller valence angles 103 +/- 5 degrees and 105 +/- 5 degrees for NbO2+ and TaO2+, respectively, and the molecular anions are more open with 120 +/- 10 degrees valence angles. Evidence is also presented for stable MO3- anion species.