Probing the Nature of the Transition-Metal-Boron Bonds and Novel Aromaticity in Small Metal-Doped Boron Clusters Using Photoelectron Spectroscopy.

Photoelectron spectroscopy combined with quantum chemistry has been a powerful approach to elucidate the structures and bonding of size-selected boron clusters (B), 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 RhBO clusters led to the discovery of the first quadruple bond between boron and a transition-metal atom, whereas a metal-boron triplebond was found in ReBO and IrBO. The ReB cluster was shown to be the first metallaborocycle with Möbius aromaticity, and the planar ReB cluster was found to exhibit aromaticity analogous to metallabenzenes.