All-metal aromaticity of <i>cyclo</i>-Bi₃^3- in diuranium and dithorium inverse-sandwich-type complexes

Delocalised [4n+2]π-aromaticity in cyclic planar unsaturated organic molecules conceptually underpins organic chemistry. Recently, the study of all-metal aromaticity has burgeoned, but whilst there has been interest in <i>cyclo</i>-{E₃} (E = P, As, Sb, Bi) species as cyclopropenium analogues, the formation of <i>cyclo</i>-{Bi₃} remains rare. Thus, the potential aromaticity of 2/6π-<i>cyclo</i>-Bi₃^+/3-as the heaviest 6p-analogue of cyclopropylium has remained open to different interpretations. Here, we report formation of diuranium and dithorium 6π-<i>cyclo</i>-Bi₃^3-inverse sandwich complexes, complementing the small number of acyclic- and cyclic-Bi_n (n = 3-5) compounds. The 6π-<i>cyclo</i>-Bi₃^3- ring exhibits significant ring currents similar to 6π-benzene, 2π-(C₃H₃)⁺, or 6π-(C₃H₃)^3-. Calculations reveal similar ring currents for 6π <i>cyclo</i>-Bi₃^3-, 2π <i>cyclo</i>-Bi₃⁺, and 0π <i>cyclo</i>-Bi₃³⁺, demonstrating<br/>dominant σ- over π-aromaticity in <i>cyclo</i>-Bi₃^3- despite the favourability of describing <i>cyclo</i>-Bi₃^3-with localised rather than delocalised bond descriptions. Confirmation of 6π-<i>cyclo</i>-Bi₃^3- σ-aromaticity provides the heaviest all-metal 6p analogue to π-aromatic (C₃H₃)^+/3-, developing organic-inorganic aromaticity benchmarking.