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In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (U VI O 2 2+ ) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom. , 2016, 27 , 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO 2 (O 2 C-CH 3 ) 2 (O 2 C-H)] - and [UO 2 (O 2 C-CH 3 )(O 2 C-H) 2 ] - . Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO 2 (H)(O 2 C-CH 3 )(O 2 C-H)] - . We also discovered that MS n CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO 2 (O)(CH 3 )] - , which reacts with H 2 O to generate [UO 2 (O)(OH)] - . DFT calculations support the observation that formation of [UO 2 (O)(OH)] - by elimination of CH 4 is favored over H 2 O addition and rearrangement to create [UO 2 (OH) 2 (CH 3 )] - .

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.