Revisiting aromaticity and stability in the diboron actinide compound Pa2B2.

Clusters composed of heavy elements, particularly actinides, provide a compelling platform for exploring unconventional bonding and the role of relativistic effects in electronic structure and stability. In this study, we critically reassess the D 2h-symmetric Pa2B2 cluster, previously claimed to exhibit double Möbius-Craig aromaticity through delocalization of 4σ and 4π electrons. Our potential energy surface (PES) analysis disproves this assignment by showing that the D 2h structure is a higher-energy isomer; the most stable form adopts a distorted tetrahedral structure. Magnetically induced current density (MICD) analysis-based on fully relativistic four-component Dirac-Coulomb calculations-further reveals the absence of a net diatropic ring current. Instead, a weak net paratropic response and a localized vortex are observed, associated with a σ Pa-Pa bond via dz2 orbitals. Multiconfigurational analysis using CASSCF(16,16) confirms that the D 2h structure is dominated by a single-reference configuration (88%), supporting the reliability of our DFT computations. As a point of contrast, we evaluated the ReB4 - cluster-experimentally observed and computationally confirmed as the global minimum-which exhibits a strong diatropic ring current (16.3 nA T-1), demonstrating that MICD reliably captures aromaticity when transition-metal d-orbitals are genuinely involved in cyclic delocalization. These findings underscore the importance of rigorous PES validation, multiconfigurational treatment, and fully relativistic analysis, including spin-orbit coupling, when assessing aromaticity in clusters of heavy elements. More broadly, this work reinforces the need to critically reassess the growing number of 'unconventional' aromatic motifs, many of which arise from incomplete analysis or mischaracterization of electronic structure rather than genuine bonding novelty.