Anionic boron clusters have been systematically investigated both experimentally and theoretically up to 30 atoms and have all been proved to be planar or quasi-planar (2D) in their global minima. However, the B26- cluster has remained elusive in this size range up to now, because of its complicated potential land-scape. Here we present a joint photoelectron spectroscopy (PES) and first-principles study on the struc-tures and bonding of this seemingly enigmatic cluster. Extensive global minimum searches, followed by high-level calculations and Gibbs free energy corrections, reveal that at least three 2D isomers, I (C1, 2A), II (C1,2A), and III (C1 , 2A), could contribute to the observed PE spectrum for the B26 - cluster. Isomer I, which has the lowest free energy at finite temperatures, is found to dominate the experimental spectrum and
represents the smallest 2D boron cluster with a hexagonal vacancy. Distinct spectral features are observed for isomer III, which has a pentagonal hole and is found to contribute to the measured PE spec-trum as a minor species. Isomer II with a close-packed triangular 2D structure, which is the global min-imum at 0 K, may also contribute to the observed spectrum as a minor species. Chemical bonding analyses show that the principal isomer I can be viewed as an all-boron analog of the polycyclic aromatic hydrocarbon C17H11+ in terms of the p bonds.
