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

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