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Abstract Understanding the chemistry of the inert actinide oxo bond in actinyl ions AnO₂²⁺is important for controlling actinide behavior in the environment, during separations, and in nuclear waste (An=U, Np, Pu). The thioether calixarene TC4A (4‐tert‐butyltetrathiacalix[4]arene) binds equatorially to the actinyl cation forming a conical pocket that differentiates the twotrans‐oxo groups. The ‘ate’ complexes, [A]₂[UO₂(TC4A)] (A=[Li(DME)₂], HNEt₃) and [HNEt₃]₂[AnO₂(TC4A)] (An=U, Np, Pu), enable selective oxo chemistry. Silylation of the U^VIoxo groups by bis(trimethylsilyl)pyrazine occurs first at only the unencapsulatedexooxo and only one silylation is needed to enable migration of theendooxo out of the cone, whereupon a second silylation affords the stable U^IVcis‐bis(siloxide) [A]₂[U(OSiMe₃)₂(TC4A)]. Calculations confirm that only one silylation event is needed to initiate oxo rearrangement, and that the putativecisdioxo isomer of [UO₂(TC4A)]²⁻would be stable if it could be accessed synthetically, at only 23 kcal.mol⁻¹in energy above the classicaltransdioxo. Calculations for the transuraniccis[AnO₂(TC4A)]²⁻(An=Np, Pu) are at higher energies, 30–35 kcal.mol⁻¹, retaining the U complexes as the more obvious target for acis‐dioxo actinyl ion. The aryloxide (OAr) groups of the macrocycle are essential in stabilizing this as‐yet unseen uranyl geometry as further bonding in the TC4A U‐O_Argroups stabilizes the U=O ‘yl’ bonds, explaining the stability of the putativecis[UO₂(TC4A)]²⁻in this ligand framework.