Decisive Role of 5f-Orbital Covalence in the Structure and Stability of Pentavalent Transuranic Oxo [M6O8] Clusters

Abstract

Actinide metal oxo clusters are of vital importance in actinide chemistry, as well as in environmental and materials sciences. They are ubiquitous in both aqueous and nonaqueous phases and play key roles in nuclear materials (e.g., nuclear fuel) and nuclear waste management. Despite their importance, our structural understanding of the actinide metal oxo clusters, particularly the transuranic ones, is very limited because of experimental challenges such as high radioactivity. Herein we report a systematic theoretical study on the structures and stabilities of seven actinide metal oxo-hydroxo clusters [An^IV₆O₄(OH)₄L₁₂] (1-An; An = Th-Cm; L = O₂CH^-) along with their group 4 (Ti, Zr, Hf, Rf) and lanthanide (Ce) counterparts [M^IV₆O₄(OH)₄L₁₂] (1-M). The work shows the T_d-symmetric structures of all of the 1-An/M clusters and suggests the positions of the -OH functional groups, which are experimentally challenging to determine. Furthermore, by removing six electrons from 1-An, we found that oxidation could happen on the An^IV metal ions, producing [An^V₆O₄(OH)₄L₁₂]⁶⁺ (2-An; An = Pa, U, Np), or on the O^2- and OH^- ligands, producing [An^IV₆(O^•-)₄(OH^•)₂(OH)₂L₁₂]⁶⁺ (3-An; An = Pu, Am, Cm). On the basis of 2-An, we constructed a series of tetravalent and pentavalent actinide metal oxo clusters [An^IV₆O₁₄]^4- (4-An) and [An^V₆O₁₄]²⁺ (5-An), which proves the feasibility of the highly important pentavalent actinyl clusters, demonstrates the f orbital's structure-directing role in the formation of linear [O≡An^V═O]⁺ actinyl ions, and expands the concept of actinyl-actinyl interaction into pentavalent transuranic actinyl clusters.