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Spontaneous symmetry-breaking is common in chemical and physical systems. Here, we show that by adding an electron to the C 7 v PbB 8 cluster, which consists of a planar B 8 disk with the Pb atom situated along the C 7 axis, the Pb atom spontaneously moves to the off-axis position in the PbB 8 - anion. Photoelectron spectroscopy of PbB 8 - reveals a broad ground-state transition and a large energy gap, suggesting a highly stable closed-shell PbB 8 borozene complex and a significant geometry change upon electron detachment. Quantum chemistry calculations indicate that the lowest unoccupied molecular orbital of the C 7 v PbB 8 cluster is a degenerate π orbital mainly consisting of the Pb 6p x and 6p y atomic orbitals. Occupation of one of the 6p orbitals spontaneously break the C 7 v symmetry in the anion due to the Jahn-Teller effect. The large amplitude of the position change of Pb in PbB 8 - relative to PbB 8 is surprising owing to bonding interactions between the Pb 6p orbital with the π orbital of the B 8 borozene.

Copper has been found to be able to mediate the formation of bilayer borophenes. Copper-boron binary clusters are ideal model systems to probe the copper-boron interactions, which are essential to understand the growth mechanisms of borophenes on copper substrates. Here, we report a joint photoelectron spectroscopy and theoretical study on two di-copper-doped boron clusters: Cu 2 B 3 - and Cu 2 B 4 - . Well-resolved photoelectron spectra are obtained, revealing the presence of a low-lying isomer in both cases. Theoretical calculations show that the global minimum of Cu 2 B 3 - ( C 2 v , 1 A 1 ) contains a doubly aromatic B 3 - unit weakly interacting with a Cu 2 dimer, while the low-lying isomer ( C 2 v , 1 A 1 ) consists of a B 3 triangle with the two Cu atoms covalently bonded to two B atoms at two vertexes. The global minimum of Cu 2 B 4 - ( D 2 h , 2 A g ) is found to consist of a rhombus B 4 unit covalently bonded to the two Cu atoms at two opposite vertexes, whereas in the low-lying isomer ( C s , 2 A '), one of the two Cu atoms is bonded to two B atoms.

Because of its low toxicity, bismuth is considered to be a "green metal" and has received increasing attention in chemistry and materials science. To understand the chemical bonding of bismuth, here we report a joint experimental and theoretical study on a series of bismuth-doped boron clusters, BiB n - ( n = 6-8). Well-resolved photoelectron spectra are obtained and are used to understand the structures and bonding of BiB n - in conjunction with theoretical calculations. Global minimum searches find that all three BiB n - clusters have planar structures with the Bi atom bonded to the edge of the planar B n moiety via two Bi-B σ bonds as well as π bonding by the 6p z orbital. BiB 6 - is found to consist of a double-chain B 6 with a terminal Bi atom. Both BiB 7 - and BiB 8 - are composed of a Bi atom bonded to the planar global minima of the B 7 - and B 8 - clusters. Chemical bonding analyses reveal that BiB 6 - is doubly antiaromatic, whereas BiB 7 - and BiB 8 - are doubly aromatic. In the neutral BiB n ( n = 6-8) clusters, except BiB 6 which has a planar structure similar to the anion, the global minima of both BiB 7 and BiB 8 are found to be half-sandwich-type structures due to the high stability of the doubly aromatic B 7 3- and B 8 2- molecular wheel ligands.

The strong relativistic effects result in many interesting chemical and physical properties for gold and gold compounds. One of the most surprising findings has been that small gold clusters prefer planar structures. Dopants can be used to tune the electronic and structural properties of gold nanoclusters. Here we report an experimental and theoretical investigation of a Zn-doped gold cluster, Au 9 Zn - . Photoelectron spectroscopy reveals that Au 9 Zn - is a highly stable electronic system with an electron binding energy of 4.27 eV. Quantum chemical studies show that the global minimum of Au 9 Zn - has a D 3 h structure with a closed-shell electron configuration ( 1 A 1 '), which can be viewed as replacing the central Au atom by Zn in the open-shell parent Au 10 - cluster. The high electronic stability of Au 9 Zn - is corroborated by its extremely large HOMO-LUMO gap of 3.3 eV. Chemical bonding analyses revealed that the D 3 h Au 9 Zn - are bonded by two sets of delocalized σ bonds, giving rise to double σ aromaticity and its remarkable stability. Two planar low-lying isomers are also observed, corresponding to a similar triangular structure with the Zn atom on the edge and another one with one of the corner Au atoms moved to the edge of the triangle.

Inverse-sandwich structures have been observed recently for dilanthanide boride clusters, in which two Ln atoms sandwich a monocyclic B x ring for x = 7-9. An interesting question is if larger B x rings are possible to form such inverse-sandwich clusters. Here we address this question by investigating La 2 B 10 - and La 2 B 11 - using photoelectron spectroscopy and ab initio quantum chemical calculations. Photoelectron spectra of La 2 B 10 - and La 2 B 11 - show complicated, but well-resolved, spectral features that are used to compare with theoretical calculations. We have found that global minimum structures of the two clusters are based on the octa-boron ring. The global minimum of La 2 B 10 - consists of two chiral enantiomers with C 1 symmetry, which can be viewed as adding a B 2 unit off-plane to the B 8 ring, whereas that of La 2 B 11 - can be viewed as adding a B 3 unit in-plane to the B 8 ring in a second coordination shell. Chemical bonding analyses reveal localized B-B bonds on the edge of the clusters and delocalized bonds in the expanded boron frameworks. The interactions between the La atoms and the boron frameworks include the unique (d-p)δ bonding, which was found to be the key for inverse-sandwich complexes with monocyclic boron rings. The current study confirms that the largest monocyclic boron ring to form the inverse-sandwich structures is B 9 and provide insights into the structural evolutions of larger lanthanide boride clusters.

Transition-metal-centered monocyclic boron wheel clusters (M©B n q ) represent a family of interesting borometallic compounds with double aromaticity. A variety of early and late transition metal atoms have been found to form such structures with high symmetries and various B n ring sizes. Here we report a combined photoelectron spectroscopy and quantum-chemistry theoretical study of two M©B n - clusters from the middle of the transition metal series: Re©B 8 - and Re©B 9 - . Global minimum structure searches revealed that ReB 8 - adopts a pseudo- C 8 v structure while ReB 9 - is a perfectly planar D 9 h molecular wheel. Chemical bonding analyses showed that both clusters exhibit σ and π double aromaticity and obey the electronic design principle for metal-centered borometallic molecular wheels. The central Re atoms are found to possess unusually low oxidation states of +I in Re©B 8 - and +II in Re©B 9 - , i.e., the Re atom behaves similarly to late transition metal elements (Ru, Fe, Co, Rh, Ir) in the M©B n - molecular wheels. These two clusters become new members of the family of transition-metal-centered monocyclic borometallic molecular wheels, which may be viable for chemical syntheses with appropriate ligands.