Laser-ablated chromium, molybdenum, and tungsten atoms react with O-2 in excess neon during condensation to form the MO2 dioxide molecules as major products. In addition, the MO2-anions, formed via capture of ablated electrons by the neutral molecules, were identified from isotopic splittings on their matrix infrared spectra and from density functional theory (DFT) calculations of isotopic frequencies. Evidence is also presented for CrO3 and the CrO3- anion. Doping with CCl4 to serve as an electron trap gave the same neutral molecules and virtually eliminated the anion absorptions, which strongly supports the anion identification.