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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. 

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d  BR back donation, despite the electron-deficiency of boron. Electron-precise metal-boron triple bond was first observed in BiB2O– [Bi≡B-B≡O]–, in which both boron atoms can be viewed as being engaged in sp hybridization and the [B-BO]– fragment is isoelectronic to a carbyne (CR). To search for the first electron-precise transition-metal-boron triple bond species, we have produced IrB2O– and ReB2O– and investigated them by photoelectron spectroscopy and quantum calculations. Well-resolved photoelectron spectra are combined with ab initio calculations to elucidate the structures and bonding in the two clusters. We find IrB2O– has a closed-shell bent structure (Cs, 1A'), with BO– coordinated to an Ir≡B unit, (–OB)Ir≡B, whereas ReB2O– is found to be linear (C∞, 3Sigma–) with an electron-precise Re≡B triple bond, [Re≡B-B≡O]–. The current result suggests the intriguing possibility of synthesizing compounds with electron-precise M≡B triple bonds analogous to the classical carbyne systems. 

We report a high-resolution photoelectron imaging and theoretical study of the IrB3‾ cluster. Two isomers are observed experimentally with electron affinities (EAs) of 1.3147(8) eV and 1.937(4) eV. Quantum calculations reveal two nearly degenerate isomers competing for the global minimum, both with a B3 ring coordinated with the Ir atom. One of the isomers consists of a B3 ring with a bridge-bonded Ir (Cs, 2A), which has the higher EA; and the second isomer features a tetrahedral structure (C3v, 2A1), which has the lower EA. The neutral tetrahedral structure is predicted to be overwhelmingly more stable than all other isomers. Chemical bonding analyses show that the neutral C3v isomer involves significant covalent Ir–B bonding and weak ionic bonding with charge transfers from B3 to Ir, which can be viewed as an Ir‾(η3-B3+) complex. The current study reports the first example of a boron-to-metal charge transfer complex and provides evidence of a π-aromatic B3+ ring coordinated to a transition metal.