We report the synthesis and characterization of a series of new, tunable 1,2-bis(diarylphosphine oxide)-ortho-carboranes, derivatives of our previously reported uranyl (UO22+) capture agent 1,2-(Ph2PO)2-1,2-C2B10H10 (POCb). The series features new cage-substituted variants of POCb, namely, 9-I-POCb (POCbI), 9,12-I2-POCb (POCbI2), 9,12-Me2-POCb (POCbMe2), 9,12-Et2-POCb (POCbEt2), and 4,5,7,8,9,10,11,12-Me8-POCb (POCbMe8). Aryl-substituted variants include 1,2-((4-MeO-Ph)2PO)2-Cb ((OMe)POCb) and 1,2-((4-F-Ph)2PO)2-Cb ((F)POCb). The effects of electron-withdrawing (EWG) and electron-donating (EDG) groups on resulting carborane redox potentials were assessed using electrochemical means, and the resulting Lewis basicities were quantified using empirical and competition-based NMR experiments. In organic solution, carboranes substituted with EWGs exhibited weaker coordination to UO22+, whereas those with EDGs exhibited stronger coordination. Similar to the previously reported unsubstituted POCb, the tunable new series of carboranes were electrochemically reduced and used for the biphasic capture of UO22+ from an aqueous to an organic phase and back again (release) through electrochemical oxidation. Extraction and back-extraction efficiencies were determined by analyses of the aqueous phases by ICP-OES. While all reduced nido-carboranes efficiently extracted UO22+ in high yields (78-88%)─with seemingly no correlation to the aforementioned measured Lewis basicities─we found the back-extraction of UO22+ to be significantly improved from POCb and, surprisingly, more closely related to their hydrophobic rather than their Lewis basic properties.