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. [ABSTRACT FROM AUTHOR]