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

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