Understanding of reaction and structural property of diuranium(III) complexes of a single ligand is essential to advancing their experimental synthesis and exploring promising applications in small molecule activation chemistry. In this work, we theoretically examined a series of diuranium(III) complexes building on the reaction of [UI3(THF)4] and octadentate polypyrrolic ligands (H4L1 and H4L2). At the relativistic density functional theory level, 11.2kcal/mol energy is required to form the Pacman-like [(UI)2(μ2-I)(L1)]− (1) in THF solution, while only 0.5kcal/mol for its non-classic Pacman isomer 1n. These agree with their analogues that have been experimentally proposed. A variety of chemical modification has been made for 1. The addition of one THF solvent or one iodine ion raises reaction energy by 6.8∼14.5kcal/mol, while two or more are significantly endoergic. Smaller energies of 1.4 and 7.4kcal/mol are required when varying bridged atom (from the iodine of 1 into the THF of 2 [(UI)2(μ2-THF)(L1)]) and lengthening ligand linker size (from ortho-phenylene of H4L1 of 1 into anthracene of H4L2 of 3 [(UI)2(μ2-I)(L2)]−), respectively. Eleven diuranium(III) complexes were optimized to be energetically stable. The U–U distances of 4.13 and 4.08Å were calculated for 1 and 2, respectively, compared with the long one of 5.90Å for 3.