Results from Car-Parrinello molecular dynamics simulations are reported for [UO2(OH2) 5](2+), UO2(NO3)(2)(OH2)(2), and UO2(NO3)(2)(eta(2)-tmma) (tmma = tetramethylmalonamide) in the gas phase and in aqueous solution. The distances between uranyl and neutral ligands such as water and tmma are decreased by up to 0.2 angstrom upon hydration, whereas those between uranyl and the nitrate ion are increased by up to 0.08 angstrom. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, solvation facilitates the transition of the chelating nitrate ligand to a eta(1)-bonding mode: the free energy of UO2(eta(2)-NO3)(eta(1)-NO3)(OH2)(2) relative to the bis-chelating minimum drops from 3.9 kcal/mol in vacuo to 1.4 kcal/mol in water. Optimizations in a polarizable continuum (specifically, the conductor-like screening model in conjunction with the zero-order regular approximation and triple. Slater basis sets) can qualitatively reproduce the geometrical changes from explicit hydration.