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The reactivity of Bi_n⁻clusters (n= 2 to 30) with O₂is found to display even-odd alternations. The open-shell even-sized Bi_n⁻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 Bi₈cages linked by a Bi₂dimer, where each atom is bonded to three neighboring atoms. Chemical bonding analyses reveal that each Bi uses its three 6pelectrons to form three covalent bonds with its neighbors, resulting in a Bi₁₈⁻cluster without any dangling bonds. We find that the robust Bi₁₈framework along with the totally delocalized unpaired electron is responsible for the surprising inertness of Bi₁₈⁻toward O₂. The Bi₁₈framework is similar to that in Hittorf’s phosphorus, suggesting the possibility to create bismuth nanoclusters with interesting structures and properties. 

Because of their interesting structures and bonding and potentials as motifs for new nanomaterials, size-selected boron clusters have received tremendous interest in recent years. In particular, boron cluster anions (B n − ) have allowed systematic joint photoelectron spectroscopy and theoretical studies, revealing predominantly two-dimensional structures. The discovery of the planar B 36 cluster with a central hexagonal vacancy provided the first experimental evidence of the viability of 2D borons, giving rise to the concept of borophene. The finding of the B 40 cage cluster unveiled the existence of fullerene-like boron clusters (borospherenes). Metal-doping can significantly extend the structural and bonding repertoire of boron clusters. Main-group metals interact with boron through s/p orbitals, resulting in either half-sandwich-type structures or substitutional structures. Transition metals are more versatile in bonding with boron, forming a variety of structures including half-sandwich structures, metal-centered boron rings, and metal-centered boron drums. Transition metal atoms have also been found to be able to be doped into the plane of 2D boron clusters, suggesting the possibility of metalloborophenes. Early studies of di-metal-doped boron clusters focused on gold, revealing ladder-like boron structures with terminal gold atoms. Recent observations of highly symmetric Ta 2 B 6 − and Ln 2 B n − ( n = 7–9) clusters have established a family of inverse sandwich structures with monocyclic boron rings stabilized by two metal atoms. The study of size-selected boron and doped-boron clusters is a burgeoning field of research. Further investigations will continue to reveal more interesting structures and novel chemical bonding, paving the foundation for new boron-based chemical compounds and nanomaterials.