New mu(3)-hydroxido/oxido bridged trinuclear uranyl-(VI) complexes with 3,5-di-t-butyl-N-salicylidene-2-aminophenolato (dbusap(2-)) ligands, Et3NH[(UO2)(3)(mu(3)-OH)(dbusap)(3)] (Et3NH[1]) and (Et3NH)(2)[(UO2)(3)(mu(3)-O) (dbusap)(3)] ((Et3NH)(2)[2]) were synthesized and characterized. Single-crystal X-ray structures of both complexes were determined. The oxygen atom on mu(3)-hydroxido center in [1](-) is sp(3) hybridized with an average U-(mu(3)-O)-U bond angle of 109.7(5)degrees; the mu(3)-oxido atom in [2](2-) is sp(2) hybridized with an average U-(mu(3)-O)-U bond angle of 118.0(10)degrees. U-(mu(3)-O) distances in [1](-) are long (average of 2.43(1) angstrom) compared with those in [2](2-) (average of 2.23(2) angstrom). The optimized geometries of the [(UO2)(3)(mu(3)-OH)](5+) core in [(UO2)(3)(mu(3)-OH)(sap)(3)](-) and the [(UO2)(3)(mu(3)-O)](4+) core in [(UO2)(3)(mu(3)-O)(sap)(3)](2-) (where sap = N-salicylidene-2-aminophenolato) from, density functional theory (DFT) calculations resemble those in [1](-) and [2](2-), respectively. The U-(mu(3)-O) bond in [2](2-) is significantly shorter than that in [1](-), because of the greater negative charge on the central mu(3)-oxido. A reversible structural conversion between [2](2-) and [1](-) was conducted by protonation and deprotonation of the mu(3)-oxid/hydroxido group. The activation enthalpy and entropy of the proton self-exchange reaction between [1](-) and [2](2-) determined from the temperature dependence of H-1 NMR coalescence are Delta H++ = 23 +/- 2 kJ mol(-1) and Delta S++ = 77 +/- 5 J K-1 mol(-1).