Photoelectron spectroscopy (PES) in combination with computational chemistry has been used systematically over the past decade to elucidate the structures and chemical bonding of size-selected boron clusters. Small boron clusters have been found to be planar or quasi-planar, consisting of a monocyclic circumference with one or more interior atoms. The propensity for planarity has been found to be a result of both σ and π electron delocalisation over the molecular plane, giving rise to concepts of σ and π multiple aromaticity. In particular, the B₃₆ cluster has been found to possess a highly stable planar structure with a central hexagonal vacancy. This finding provides the first indirect experimental evidence that single-atom layer boron-sheets with hexagonal vacancies, dubbed ‘borophene’, are potentially viable. Another exciting discovery has been the observation and characterisation of the first all-boron fullerenes. PES revealed that the cluster consisted of two isomers with very different electron binding energies. Global minimum searches led to two nearly degenerate isomers competing for the global minimum: a quasi-planar isomer with a double hexagonal vacancy and an unprecedented cage isomer. In the neutral, the B₄₀ cage is overwhelmingly the global minimum, which is the first all-boron fullerene to be observed and is named ‘borospherene’. Rapid progresses in our understanding of the structures and bonding of size-selected boron clusters have been made during the past decade, which will be the focus of this review. The recent findings about borophenes and borospherenes have stimulated growing interests in boron clusters and will accelerate the pace of discovery in boron chemistry and nanostructures.

Because of their interesting structures and bonding and potentials as motifs for new nanomaterials, size-selected boron clusters have received tremendous interest in recent years. In particular, boron cluster anions (B_n ⁻) have allowed systematic joint photoelectron spectroscopy and theoretical studies, revealing predominantly two-dimensional structures. The discovery of the planar B₃₆ cluster with a central hexagonal vacancy provided the first experimental evidence of the viability of 2D borons, giving rise to the concept of borophene. The finding of the B₄₀ cage cluster unveiled the existence of fullerene-like boron clusters (borospherenes). Metal-doping can significantly extend the structural and bonding repertoire of boron clusters. Main-group metals interact with boron through s/p orbitals, resulting in either half-sandwich-type structures or substitutional structures. Transition metals are more versatile in bonding with boron, forming a variety of structures including half-sandwich structures, metal-centered boron rings, and metal-centered boron drums. Transition metal atoms have also been found to be able to be doped into the plane of 2D boron clusters, suggesting the possibility of metalloborophenes. Early studies of di-metal-doped boron clusters focused on gold, revealing ladder-like boron structures with terminal gold atoms. Recent observations of highly symmetric Ta₂B₆ ⁻ and Ln₂B_n ⁻ (n = 7–9) clusters have established a family of inverse sandwich structures with monocyclic boron rings stabilized by two metal atoms. The study of size-selected boron and doped-boron clusters is a burgeoning field of research. Further investigations will continue to reveal more interesting structures and novel chemical bonding, paving the foundation for new boron-based chemical compounds and nanomaterials.

The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric cobalt-centered boron drum-like structures of CoB₁₆⁻, characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D_8d (I) and C_4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B₈ rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB₁₆⁻ cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.