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 from C_s to C_7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C_7v-LnB₈^- clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈^2- doubly aromatic ligand. The B₇^3-, B₈^2-, 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.

While boron forms a wide range of metal borides with important industrial applications, there has been relatively little attention devoted to lanthanide boride clusters. Here we report a joint photoelectron spectroscopy and quantum chemical study on two octa-boron di-lanthanide clusters, Ln₂B₈^- (Ln = La, Pr). We found that these clusters form highly stable inverse sandwich structures, [Ln-B₈-Ln]^-, with strong Ln and B₈ bonding via interactions between the Ln 5d orbitals and the delocalized σ and π orbitals on the B₈ ring. A (d-p)δ bond, involving the 5dδ and the antibonding π orbital of the B₈ ring, is observed to be important in the Ln-B₈ interactions. The highly symmetric inverse sandwich structures are overwhelmingly more stable than any other isomers. Upon electron detachment, the (d-p)δ orbitals become half-filled, giving rise to a triplet ground state for neutral La₂B₈ In addition to the two unpaired electrons in the (d-p)δ orbitals upon electron detachment, the neutral Pr₂B₈ complex also contains two unpaired 4f electrons on each Pr center. The six unpaired spins in Pr₂B₈ are ferromagnetically coupled to give rise to a septuplet ground state. The current work suggests that highly magnetic Ln…B₈…Ln inverse sandwiches or 1D Ln…B₈…Ln nanowires may be designed with novel electronic and magnetic properties.

Despite the importance of bulk lanthanide borides, nanoclusters of lanthanide and boron have rarely been investigated. Here we show that lanthanide-boron binary clusters, La2B 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.

Five CHB(11)X(6)Y(5)(-) carborane anions from the series X = Br, Cl, I and Y = H, Cl, CH(3) were generated by electrospray ionization, and their reactivity with a series of Brønsted acids and electron transfer reagents were examined in the gas phase. The undecachlorocarborane acid, H(CHB(11)Cl(11)), was found to be far more acidic than the former record holder, (1-C(4)F(9)SO(2))(2)NH (i.e., DeltaH degrees (acid) = 241 +/- 29 vs 291.1 +/- 2.2 kcal mol(-1)) and bridges the gas-phase acidity and basicity scales for the first time. Its conjugate base, CHB(11)Cl(11)(-), was found by photoelectron spectroscopy to have a remarkably large electron binding energy (6.35 +/- 0.02 eV) but the value for the (1-C(4)F(9)SO(2))(2)N(-) anion is even larger (6.5 +/- 0.1 eV). Consequently, it is the weak H-(CHB(11)Cl(11)) BDE (70.0 kcal mol(-1), G3(MP2)) compared to the strong BDE of (1-C(4)F(9)SO(2))(2)N-H (127.4 +/- 3.2 kcal mol(-1)) that accounts for the greater acidity of carborane acids.

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb₂Au₆ ^- cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb₂Au₆ ^-/0 possess high-symmetry D _6h structures featuring a Nb-Nb axis coordinated equatorially by an Au₆ ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb₂ moiety in Nb₂©Au₆, as well as five totally delocalized σ bonds. The Nb[triple bond, length as m-dash]Nb triple bond is strengthened significantly by the delocalized σ bonds, resulting in an extremely short Nb-Nb bond length comparable to the quintuple bond in gaseous Nb₂. The totally delocalized σ bonding in Nb₂©Au₆ is reminiscent of σ aromaticity, representing a new bonding mode in metal-ligand systems. The unusually short Nb-Nb bond length in Nb₂©Au₆ shows that the Au₆ ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers via delocalized σ bonding.

Metal-doped boron clusters provide new opportunities to design nanoclusters with interesting structures and bonding. A cobalt-doped boron cluster, CoB₁₈^-, has been observed recently to be planar and can be viewed as a motif for metallo-borophenes, whereas the D_9d drum isomer as a motif for metallo-boronanotubes is found to be much higher in energy. Hence, whether larger doped boron drums are possible is still an open question. Here we report that for RhB₁₈^- the drum and quasi-planar structures become much closer in energy and co-exist experimentally, revealing a competition between the metallo-boronanotube and metallo-borophene structures. Photoelectron spectroscopy of RhB₁₈^- shows a complicated spectral pattern, suggesting the presence of two isomers. Quantum chemistry studies indicate that the D_9d drum isomer and a quasi-planar isomer (C_s) compete for the global minimum. The enhanced stability of the drum isomer in RhB₁₈^- is due to the less contracted Rh 4d orbitals, which can have favorable interactions with the B₁₈ drum motif. Chemical bonding analyses show that the quasi-planar isomer of RhB₁₈^- is aromatic with 10 π electrons, whereas the observed RhB₁₈^- drum cluster sets a new record for coordination number of eighteen among metal complexes. The current finding shows that the size of the boron drum can be tuned by appropriate metal dopants, suggesting that even larger boron drums with 5d, 6d transition metal, lanthanide or actinide metal atoms are possible.

The discovery of borospherenes unveiled the capacity of boron to form fullerene-like cage structures. While fullerenes are known to entrap metal atoms to form endohedral metallofullerenes, few metal atoms have been observed to be part of the fullerene cages. Here we report the observation of a class of remarkable metallo-borospherenes, where metal atoms are integral parts of the cage surface. We have produced La₃B₁₈^- and Tb₃B₁₈^- and probed their structures and bonding using photoelectron spectroscopy and theoretical calculations. Global minimum searches revealed that the most stable structures of Ln₃B₁₈^- are hollow cages with D_3h symmetry. The B₁₈-framework in the Ln₃B₁₈^- cages can be viewed as consisting of two triangular B₆ motifs connected by three B₂ units, forming three shared B₁₀ rings which are coordinated to the three Ln atoms on the cage surface. These metallo-borospherenes represent a new class of unusual geometry that has not been observed in chemistry heretofore.

We report a joint photoelectron spectroscopy and theoretical investigation of the gaseous Au2I3- cluster, which is found to exhibit two types of isomers due to competition between Au-I covalent bonding and Au-Au aurophilic interactions. The covalent bonding favors a bent IAuIAuI- structure with an obtuse Au-I-Au angle (100.7°), while aurophilic interactions pull the two Au atoms much closer, leading to an acutely bent structure (72.0°) with an Au-Au distance of 3.08 Å. The two isomers are separated by a small barrier and are nearly degenerate with the obtuse isomer being slightly more stable. At low temperature, only the obtuse isomer is observed; distinct experimental evidence is observed for the co-existence of a combination of isomers with both acute and obtuse bending angles at room temperature. The two bond-bending isomers of Au2I3- reveal a unique example of one molecule being able to oscillate between different structures as a result of two competing chemical forces.