Observation of a metal-centered B2-Ta@B18 tubular molecular rotor and a perfect Ta@B20 boron drum with the record coordination number of twenty    

    A tubular molecular rotor B₂-Ta@B₁₈ ^- (1) and boron drum Ta@B₂₀^- (2) with the highest coordination number of twenty in chemistry are observed via a joint photoelectron spectroscopy and first-principles theory investigation.