Structures of La(III) and Ba(II) complexes and their stabilities were investigated to develop a stable chelating ligand for Ac(III) and Ra(II) which are of interest in nuclear medicine using alpha particles. The La(III) and Ba(II) complexes (1 and 2) with a diaza-18-crown-6 ether bearing pyridine phosphonic acid ethyl ester pendant arms (L1) and the complexes (3 and 4) with a diaza-18-crown-6 ether bearing pyridine phosphonate pendant arms ((H2L2)2−) were synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. The La(III) complexes, 1 and 3, exhibited 11-coordinate structures, whereas the Ba(II) complexes, 2 and 4 exhibited 11- and 10-coordinate geometries, respectively. Potentiometric titrations were performed to determine the acid–base equilibrium and the complex formation constants of the La(III) and Ba(II) complexes. In addition, the speciation analysis of the complexes was performed and the pH dependencies of the fractions in the aqueous solutions were investigated. For the La(III) complexes, the species with the ligand bearing pyridine phosphonate pendant arms were observed to be more stable than those with the ligand bearing pyridine phosphonate ester pendant arms. The Ba(II) complexes showed the opposite trend. The species with the pyridine phosphonate ester ligand were observed to be more stable than the species with the pyridine phosphonate type ligand.