All-metal aromaticity of cyclo-Bi33- in diuranium and dithorium inverse-sandwich-type complexes

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