The hydrothermal synthesis and structures Of [UO2(PDA)] (1) and [Th(PDA)(2)(H2O)(2)] center dot H2O (2) (PDA = 1,10-phenanthroline-2,9-dicarboxylic acid) are reported. 1 is orthorhombic, Pnma, a = 11.1318(7) angstrom, b = 6.6926(4) angstrom, c = 17.3114(12) angstrom, V = 1289.71(14), Z = 4, R = 0.0313; 2 is triclinic, P (1) over bar, a = 7.6190(15) angstrom, b 10.423(2) angstrom, c = 17.367(4) angstrom, alpha = 94.93(3)degrees, beta = 97.57(3)degrees, gamma = 109.26(3)degrees, V = 1278.3(4) angstrom(3), Z = 2, R = 0.0654. The local geometry around the U in 1 is a pentagonal bipyramid with the two uranyl oxygens occupying the apical positions. The donor atoms in the plane comprise the four donor atoms from the PDA ligand (average U-N = 2.558 and U-O = 2.351 angstrom) with the fifth site occupied by a bridging carboxylate oxygen from a neighboring UO2/PDA individual. The PDA ligand in 1 is exactly planar, with the U lying in the plane of the ligand. The latter planarity, as well as the near-ideal U-O and U-N bond lengths, and O-U-N and N-U-N bond angles within the chelate rings of 1 suggest that PDA binds to the uranyl cation in a low-strain manner. In 2, there are two PDA ligands bound to the Th (average Th-N = 2.694 and Th-O = 2.430 angstrom) as well as two water molecules (Th-O = 2.473 and 2.532 A) to give the Th a coordination number of 10. The PDA ligands in 2 are bowed, with the Th lying out of the plane of the ligand. Molecular mechanics calculations suggest that the distortion of the PDA ligands in 2 arises because of steric crowding. UV spectroscopic studies of solutions containing 1:1 ratios of PDA and Th4+ in 0.1 M NaClO4 at 25 degrees C indicate that log K, for the Th4+/PDA complex is 25.7(9). The latter result confirms the previous prediction that complexes of PDA with metal ions of higher charge and an ionic radius of about 1.0 angstrom such as Th(IV) would have remarkably high log K, values with PDA. The origins of this very high stability are discussed in terms of a synergy between the pyridyl and the carboxylate donor groups of PDA. Metal ions of high charge normally bond poorly with pyridyl donors in aqueous solution because such metal ions require donor groups that are able to disperse charge to the solvent via hydrogen-bonding, which pyridyl groups are unable to do. In PDA, the carboxylates fulfill this need and so enable the high donor strength of the pyridyl groups of PDA to become apparent in the high log K-1 for Th(IV) with PDA.