Mobius aromaticity in bipyramidal rhodium-centered bismuth clusters

The chemical bonding topologies in the mixed rhodium–bismuth clusters [RhBi6(µ-X)12]3–(X = Br, I) and RhBi7Br8 are compared. The octahedral cluster [RhBi6(µ-X)12]3– can be considered as a d6 Rh(III) complex containing six equivalent BiX2– ligands with interligand halogen bridging but no Bi–Bi bonding. However, the pentagonal bipyramidal cluster RhBi7Br8 has only two similar BiBr2– ligands in the axial positions. The five equatorial bismuth atoms in RhBi7Br8 form a bonding pentagon, which is an example of a five-atom M?bius aromatic system with a fivecenter four-electron bond. This unusual bond involves -type overlap between p orbitals in the plane of the pentagon with the single phase change required for M?bius aromaticity.