We report a comparison of the molecular and electronic structures of dithio- and diselenophosphinate, (E2PR2)(1-)(E = S, Se; R = Pr-i, Bu-t), with thorium(IV) and uranium(IV) complexes. For the thorium dithiophosphinate complexes, reaction of ThCI4(DME)(2) with 4 equiv of KS2PR2 (R = Pr-i, Bu-t) produced the homoleptic complexes, Th((S2PPr2)-Pr-i)(4) (IS-Th-Pr-i) and Th((S2PBu2)-Bu-t)(4) (2S-Th-Bu-t). The diselenophosphinate complexes were synthesized in a similar manner using KSe2PR2 to produce Th((Se2PPr2)-Pr-i)(4) (1Se-Th-Pr-i) and Th(Se(2)rBu(2))(4) (2Se-Th-Bu-t). U((S2Pr2)-Pr-i)(4), 1S-U-Pr-i, could be made directly from UCI4 and 4 equiv of (KS2PPr2)-Pr-i. With ((Se2PPr2)-Pr-i)(1-), using UCI4 and 3 or 4 equiv of (KSe2PPr2)-Pr-i yielded the monochloride product U((Se2PPr2)-Pr-i)(3)C1 (3Se-U-ipr-C1), but using UI4(1,4-dioxane)(2) produced the homoleptic U((Se2PPr2)-Pr-i)(4) (1Se-(UPr)-Pr-i). Similarly, the reaction of UCI4 with 4 equiv of (KS2PBu2)-Bu-i yielded U((S2PBu2)-Bu-i)4 (2S-U-Bu-t), whereas the reaction with (KSe2PBu2)-Bu-i resulted in the formation of U((Se2PBu2)-Bu-t)3C1 (4Se-U-tBu-C1). Using UI4(1,4-dioxane)(2) and 4 equiv of (KSe2PBu2)-Bu-t with UCI4 in acetonitrile yielded U(Se2rBu2)4 (2Se-U-Bu-t). Transmetalation reactions were investigated with complex 2Se-U-tBu and various CuX (X = Br, I) salts to yield U(Se2rBu2)(3)X(6Se-(UBu)-Bu-t-Br and 7Se-U-tBu-I) and 0.25 equiv of [Cu(Se2rBu2)](4) (8Se-Cu-Bu-t). Additionally, 2Se-U-Bu-t underwent transmetalation reactions with Hg2F2 and ZnCl2 to yield U((SePBu2)-Bu-t)(3)F (6) and U((Se2PBu2)-Bu-t)(3)C1 (4Se-(UCI)-C-tBU), respectively. The molecular structures were analyzed using H-1,, C-13, P-31, and Se-77 NMR and IR spectroscopy and structurally characterized using X-ray crystallography. Using the QTAIM approach, the electronic structure of all homoleptic complexes was probed, showing slightly more covalent bonding character in actinide selenium bonds over actinide sulfur bonds.