On the remarkable resistance to oxidation by the Bi₁₈^- cluster

The reactivity of Bi^- clusters (n = 2 to 30) with O₂ is found to display even-odd alternations. The open-shell even-sized Bi^- clusters are more reactive than the closed-shell odd-sized clusters, except Bi₁₈^-, which exhibits no observable reactivity toward O₂. We have investigated the structure and bonding of Bi₁₈^- to understand its remarkable resistance to oxidation. We find that the most stable structure of Bi₁₈^- consists of two Bi₈ cages linked by a Bi₂ dimer, where each atom is bonded to three neighboring atoms. Chemical bonding analyses reveal that each Bi uses its three 6p electrons to form three covalent bonds with its neighbors, resulting in a Bi₁₈^- cluster without any dangling bonds. We find that the robust Bi₁₈ framework along with the totally delocalized unpaired electron is responsible for the surprising inertness of Bi₁₈^- toward O₂. The Bi₁₈ framework is similar to that in Hittorf's phosphorus, suggesting the possibility to create bismuth nanoclusters with interesting structures and properties.