Stable complexes of pentavalent uranyl [UO2(salan-Bu-t(2))(py)K](n) (3), [UO2(salan-Bu-t(2))(py)K(18C6)] (4), and [UO2(salophen-Bu-t(2))(thf)]K(thf)(2)}(n), (8) have been synthesized from the reaction of the complex {[UO(2)py(5)][KI(2)py(2)]}(n) (1) with the bulky amine-phenolate ligand potassium salt K-2(salan-Bu-t(2)) or the Schiff base ligand potassium salt K-2(salophen-Bu-t(2)) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [(UO2)-O-v(salan-Bu-t(2))(py)][Cp-2*Co], 6, were also isolated from the reduction of [(UO2)-O-VI(salan-Bu-t(2))(py)], 5, with Cp-2*Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO(2)py(5)]I.py (2). The measured redox potentials E-1/2 (U-VI/U-V) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give mu(eff) of 2.20 and 2.23 mu(B) at 300 K respectively, which is compared with a mu(eff) of 2.6(1) mu(B) (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g(x) = 1.98; g(y) = 1.25; g(z) = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the pi(u)*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO2(salan-Bu-t(2))(py)K] and [UO2(salophen-Bu-t(2))(py)K] complexes with respect to disproportionation and hydrolysis.