The synthesis, reactivity, structures, and bonding in gas -phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, An(v)O(2)(C2O4)(-), where An = Pa or U, are essentially actinyl ions, An(v)O(2)(+), coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, An(v)O(OH)(2)(C2O4)(-). The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas Pa-VI is inaccessible, U-VI is very stable. The (UO2)-O-v(C2O4)(-) complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O-2 to yield UO4 and two CO2 molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, (UO22+)-O-vI, coordinated in the equatorial plane by two equivalent 0 atoms each having formal charges of -1.5 and U-O bond orders intermediate between single and double. The unreactive nature of (PaO2)-O-v(C2O4)(-) toward O-2 is a manifestation of the resistance toward oxidation of Pay, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO4-, reacts with water to yield UO5H2,. Infrared spectra obtained for UO5H2- confirm the computed lowest -energy structure, UO3(OH)(2)(-).