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Abstract The study of actinide electronic structure and bonding within rigorously controlled environments is fundamental to advancing nuclear applications. Here, we report a new set of isostructural actinide organometallics; An(COT^big)₂, (An = Th, U, Np, and Pu), where COT^bigis the bulky 1,4-bis(triphenylsilyl)-substituted cyclooctatetraenyl dianion (1,4-(Ph₃Si)₂C₈H₆)^2-. The actinide(IV) metallocene sandwiches have a clam-shell structure, offering a new molecular symmetry to exploref-orbital contributions in bonding. Combined experimental and computational studies reveal that An(COT^big)₂complexes strongly differ from the previously published coplanar An(COT)₂sandwiches due to the bent geometry and electron-withdrawing nature of the substituents. While COT^bigdisplays comparatively weaker electron donation, the low-energyf-ftransitions in An(COT^big)₂have increased molar absorptivity consistent with the removal of the parity selection rule and better energetic matching between ligand and actinide 5forbitals as the series is traversed. For Pu(COT^big)₂, covalent mixing of donor 5fmetal orbitals and the ligand-π orbitals is especially strong. 

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.