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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. 

The concept of metalla-aromaticity proposed by Thorn–Hoffmann ( Nouv. J. Chim . 1979, 3, 39) has been expanded to organometallic molecules of transition metals that have more than one independent electron-delocalized system. Lanthanides, with highly contracted 4f atomic orbitals, are rarely found in multiply aromatic systems. Here we report the discovery of a doubly aromatic triatomic lanthanide-boron molecule PrB 2 − based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB 2 − has a C 2v triangular structure with a paramagnetic triplet 3 B 2 electronic ground state, which can be viewed as featuring a trivalent Pr(III,f 2 ) and B 2 4− . Chemical bonding analyses show that this cyclo-PrB 2 − species is the smallest 4f-metalla-aromatic system exhibiting σ and π double aromaticity and multiple Pr–B bonding characters. It also sheds light on the formation of the rare B 2 4− tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B 2 4− , C 2 2− , N 2 , and O 2 2+ series. 

Photoelectron spectroscopy and quantum chemistry studies are used to investigate the structure and bonding of AuB 8 − . Global minimum sturctural searches show that AuB 8 − possesses a chair-like structure, which can be viewed as Au + bonded to the edge of the doubly-aromatic B 8 2− borozene, Au + [η 2 -B 8 2− ]. Chemical bonding analyses reveal that the AuB 8 − is a novel borozene complex with unique Au–borozene bonding. 

Abstract Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB₈⁻, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB₈⁻species changes fromC_stoC_7vsymmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All theC_7v-LnB₈⁻clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈²⁻doubly aromatic ligand. The B₇³⁻, B₈²⁻, and B₉⁻series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C₅H₅⁻, C₆H₆, and C₇H₇⁺, respectively, with similar trends of size and charge state and they are named collectively as “borozenes”. Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes. 

Despite the importance of bulk lanthanide borides, nanoclusters of lanthanide and boron have rarely been investigated. Here we show that lanthanide–boron binary clusters, La 2 B x − , can form a new class of inverse-sandwich complexes, [Ln(η x -B x )Ln] − ( x = 7–9). Joint experimental and theoretical studies reveal that the monocyclic B x rings in the inverse sandwiches display similar bonding, consisting of three delocalized σ and three delocalized π bonds. Such monocyclic boron rings do not exist for bare boron clusters, but they are stabilized by the sandwiching lanthanide atoms. An electron counting rule is proposed to predict the sizes of the B x ring that can form stable inverse sandwiches. A unique (d-p)δ bond is found to play important roles in the stability of all three inverse-sandwich complexes.