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Photoelectron spectroscopy combined with quantum chemistry has been a powerful approach to elucidate the structures and bonding of size-selected boron clusters (B n − ), revealing a prevalent planar world that laid the foundation for borophenes. Investigations of metal-doped boron clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The current review focuses on recent advances in transition-metal-doped boron clusters, including the discoveries of metal-boron multiple bonds and metal-doped novel aromatic boron clusters. The study of the RhB − and RhB 2 O − clusters led to the discovery of the first quadruple bond between boron and a transition-metal atom, whereas a metal-boron triple bond was found in ReB 2 O − and IrB 2 O − . The ReB 4 − cluster was shown to be the first metallaborocycle with Möbius aromaticity, and the planar ReB 6 − cluster was found to exhibit aromaticity analogous to metallabenzenes. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

Since the discovery of the B 40 borospherene, research interests have been directed to the structural evolution of even larger boron clusters. An interesting question concerns if the borospherene cages persist in larger boron clusters like the fullerenes. Here we report a photoelectron spectroscopy (PES) and computational study on the structures and bonding of B 41 − and B 42 − , the largest boron clusters characterized experimentally thus far. The PE spectra of both clusters display broad and complicated features, suggesting the existence of multiple low-lying isomers. Global minimum searches for B 41 − reveal three low-lying isomers ( I–III ), which are all related to the planar B 40 − structure. Isomer II ( C s , 1 A′) possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I ( C s , 3 A′′) and III ( C s , 1 A′) both with a single hexagonal vacancy are also present as minor isomers in the experiment. The potential landscape of B 42 − is found to be much more complicated with numerous low-lying isomers ( VII–XII ). The quasi-planar structure VIII ( C 1 , 2 A) containing a double hexagonal vacancy is found to make major contributions to the observed PE spectrum of B 42 − , while the other low-lying isomers may also be present to give rise to a complicated spectral pattern. Chemical bonding analyses show isomer II of B 41 − ( C s , 1 A′) and isomer VIII of B 42 − ( C 1 , 2 A) are π aromatic, analogous to that in the polycyclic aromatic hydrocarbon C 27 H 13 + ( C 2v , 1 A 1 ). Borospherene cage isomers are also found for both B 41 − and B 42 − in the global minimum searches, but they are much higher energy isomers. 

Among the series of stable closo -borate dianions, [B n H n ] 2− , the X-ray crystallographic structure of [B 7 H 7 ] 2− was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp 2 M) 2 B 9 H 11 ] (Cp = η 5 -C 5 H 5 ; M = Zr or Hf). The structures of [(Cp 2 M) 2 B 9 H 11 ] contain a pentagonal bipyramidal B 7 core, coordinated by two {Cp 2 M} and two {BH 2 } units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp 2 M) 2 B 9 H 11 ] complexes are substantially more stable than the parent dianion, in either [B 7 H 7 ] 2− or ( n Bu 4 N) 2 [B 7 H 7 ]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo -{Cp 2 M} units, as well as electrostatic interactions between the {Cp 2 M} units and the B 7 core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.