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Stable complexes of pentavalent uranyl [UO2(salan-tBu2)(py)K)n(3), [UO2(salan-tBu2)(py)K(18C6)] (4), and [UO2(salophen-tBu2)(thf)]K(thf)2)n (8) have been synthesized from the reaction of the complex ([UO2py5][Kl2py2])n (1) with the bulky amine-phenolate ligand potassium salt K2(salan-tBu2) or the Schiff base ligand potassium salt K2(salophen-tBu2) in pyridine. They were characterized by NMR, Rl, 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 [UVIO2(salan-tBu2)(py)][Cp*2Co], 6, were also isolated from the reduction of [UIVO2(salan-tBu2)(py)], 5, with Cp*2Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO2py5]l.py (2). The measured redox potentials E1/2 (UVI/UV) 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 μeff of 2.20 and 2.23 μB at 300 K respectively, which is compared with a μeff of 2.6(1) μB (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (gx = 1.98; gy = 1.25; gz = 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 πu. The presence of a back-bonding interaction is likely to play. a role in the observed solution stability of the [UO2(salan-tBu2)(py)K] and [UO2(salophen-tBu2)(py)K) complexes with respect to disproportionation and hydrolysis. [ABSTRACT FROM AUTHOR]