This thesis investigates the equilibrium, structural, and kinetic properties of M(III) complexes with the macrocyclic ligand OPC2A, focusing on Sc(III), Ga(III), In(III), and Tl(III). The OPC2A ligand, a modified DOTA analog, was designed to enhance rigidity and kinetic inertness. The study explored the formation and dissociation of parent complexes, [M(OPC2A)]⁺, and mixed complexes, [M(OPC2A)X], where X represents hydroxide, fluoride, iodide, or chloride ions. These complexes are considered for potential applications in medical imaging and therapy, particularly with radioisotopes like ¹⁸F and ⁴⁴Sc. Key findings include: Scandium(III) Complexes: Stable [Sc(OPC2A)]⁺ complexes were formed with a high thermodynamic stability (log K = 16.72(4)). The dissociation kinetics in acidic solutions indicated significant inertness, with a half-life of 0.37 hours in 1 M HCl, and an exceptionally long half-life at physiological pH. The formation of mixed hydroxo [Sc(OPC2A)(OH)] and fluoro [Sc(OPC2A)F] complexes was also characterized, with the latter showing remarkable stability (log K = 4.54(4)) and moderate inertness against fluoride exchange. Radiolabeling studies with ⁴⁴Sc demonstrated high radiochemical purity and stability in serum. Additionally, a related ligand, PC2AAMnBu, was investigated, showing strong interaction with fluoride but significantly slower fluoride exchange rates compared to OPC2A. Group 13 Metal Complexes: Aluminum(III): Al(III) did not form a stable complex with OPC2A under the experimental conditions, likely due to size incompatibility and hydrolysis. Gallium(III) and Indium(III): Stable parent complexes, [Ga(OPC2A)]⁺ and [In(OPC2A)]⁺, were formed with thermodynamic stability constants of log K = 17.2(2) and 17.8(1), respectively. These complexes exhibited high inertness against acid-assisted dissociation, comparable to or better than the scandium complex. Mixed fluoro complexes, [Ga(OPC2A)F] and [In(OPC2A)F], were formed with stability constants of log K ≈ 3.5(1) and 2.9(2), respectively. Thallium(III): The [Tl(OPC2A)]⁺ complex demonstrated exceptionally high stability (estimated log K ≥ 34). Mixed complexes with iodide, [Tl(OPC2A)I], were also characterized (log K ≈ 6.0(1)). Structural studies using X-ray diffraction revealed complex solid-state structures for thallium, including [Tl(OPC2A)Cl] and [Tl₂H(OPC2A)₂Cl₂][TlCl₄]. Overall, the OPC2A ligand proved to be a versatile platform for complexing various M(III) ions, forming stable and kinetically inert complexes with potential for biomedical applications. The study highlights the importance of ligand design in achieving desired properties for radiopharmaceuticals.