The coordination chemistry of actinide(IV) ions with hydroxypyridinone ligands has been initially explored by examining the complexation of Th(IV) ion with bidentate PR-1,2-HOPO (HL1), PR-Me-3,2-HOPO (HL2), and PR-3,4-HOPO-N (HL3) ligands, The complexes Th(L-1)(4), Th(L-2)(4), and Th(L-3)(4) were prepared in methanol solution from Th(acac)(4) and the corresponding ligand. Single-crystal X-ray diffraction analyses are reported for the free ligand PR-Me-3,2-HOPO (HL2) [P (1) over bar, Z = 8, a = 8.1492(7) Angstrom, b = 11.1260(9) Angstrom, c = 23.402(2) Angstrom, alpha = 87.569(1)degrees, beta = 86.592(1)degrees, gamma = 87.480(1)degrees], and the complex Th(L-2)(4).H2O [Pna2(1) (No. 33), Z = 4, a =: 17.1250(5) Angstrom, b = 12.3036(7) Angstrom, c 23.880 (1) Angstrom]. A comparison of the structure of the metal complex Th-PR-Me-3,2-HOPO with that of free ligand PR-Me-3,2-HOPO reveals that the ligand geometry is the same in the free ligand and in the metal complex. Amide hydrogen bonds enhance the rigidity and stability of the complex and demonstrate that the Me-3,2-HOPO ligands are predisposed for metal chelation. Solution thermodynamic studies determined overall formation constants (log beta(140)) for Th(L-1)(4), Th(L-2)(4), and Th(L-3)(4) of 36.0(3), 38.3(3), and 41,8(5), respectively. Species distribution calculations show that the 4:1 metal complex Th(L)4 is the dominant species in the acidic range (pH < 6) for PR-1,2-HOPO, in weakly acidic to physiological pH range for PR-Me-3,2-HOPO and in the high-pH range (>8) for PR-3,4-HOPO-N. This finding parallels the relative acidity of these structurally related ligands. In the crystal of [Th(L-2)(4)].H2O, the chiral complex forms an unusual linear coordination polymer composed of Iiiked, alternating enantiomers.