Boron nanoclusters and few‐layer borophenes have received considerable attention in recent years due to their unique structural and bonding patterns. Based on extensive global searches and density‐functional theory calculations, we present herein the possibility of a new series of bilayer medium‐sized boron clusters including C2 B54 (I), C2h B60 (II), and C1 B62 (III) in a universal structural pattern, with one, two, and three B6 hexagonal windows on the waist around a B38 bilayer hexagonal prism at the center, respectively. Detailed orbital and bonding analyses indicate that these three‐dimensional aromatic bilayer clusters follow the σ + π double delocalization bonding pattern, with three or four effective interlayer B–B σ‐bonds formed to further stabilize the system. The IR, Raman, and UV/Vis spectra of the bilayer species are theoretically simulated to facilitate their future spectral characterizations.

Based upon global searches and electronic structure calculations at the B3LYP and CCSD(T) levels, we present the global-minimum structures of two ternary B-O-H and B-S-H rhombic clusters: D2h B2O2H2 (1, (1)Ag) and C2v B2S2H2 (2, (1)A1). Both species feature a B2X2 (X = O or S) four-membered ring as the core, with two H atoms attached terminally. The former cluster is perfectly planar, whereas the latter undergoes a slight butterfly distortion. Bonding analyses reveal a four-center four-electron (4c-4e) o-bond in these clusters, which are 4π systems in a nonbonding/bonding combination, in contrast to an antibonding/bonding combination in a classical 4π antiaromatic hydrocarbon such as cyclobutadiene (C4H4). Clusters 1 and 2 are considered to be aromatic. The present results also help elucidate the bonding nature in the relevant heteroatomic ring B2N2H4 system and suggest that it is not appropriate to consider B2N2H4 as an inorganic cyclobutadiene, a conception that has been in existence in the literature for over 40 years. The electronic properties of the global-minimum clusters 1 and 2 are predicted. It is shown that B2O2H2 (1) and B2S2H2 (2) may serve as effective inorganic ligands to form sandwich-type transition metal complexes, such as D2d [B2O2H2]2Ni (3) and D2d [B2S2H2]2Ni (4).

The successive experimental observations of planar, cage-like, seashell-like, and bilayer B n -/0 clusters in the size range between n = 3-48 well demonstrate the structural diversity and rich chemistry of boron nanoclusters. Based on extensive global minimum search and density functional theory calculations, we predict herein the bilayer C 1 B 50 (I), C 2h B 52 (II), C 1 B 56 (IV), and C 2v B 58 (V) as the global minima of the systems to ll in the missing gap in the bilayer B 2n series between B 48 -B 72 . These highly stable species all contain a B 38 bilayer hexagonal prism at the center, with 2, 2, 3, and 3 effective interlayer B-B σ-bonds formed between inward-buckled atoms on the top and bottom layers, respectively. Our bilayer C 1 B 50 (I) and C 1 B 56 (IV) prove to be obviously more stable than the previously reported monolayer planar C 2v B 50 and C 2v B 56 with two adjacent B 6 hexagonal holes. Detailed bonding analyses indicate that these bilayer clusters follow the universal bonding pattern of σ + π double delocalization, making them threedimensionally aromatic in nature. The bilayer B 2n species in the size range between B 48 -B 72 evolve gradually on the waist around the B 38 or elongated B 46 bilayer hexagonal prism at the center.

Recent experimental observation of the first bilayer clusters B 48 À /0 reveals a new structural domain in boron nanostructures. Inspired by the previously reported bilayer B 48 , B 54 , B 60 , and B 62 and based on extensive global-minimum searches and densityfunctional theory calculations, we predict herein a new series of medium-sized bilayer boron nanoclusters C 2 B 64 (I), D 2 B 66 (II), D 2 B 68 (III), C 1 B 70 (IV), and C i B 72 (V) which all contain an elongated B 46 bilayer hexagonal prism at the center with four effective interlayer 2c-2e BÀ B σ bonds formed between the top and bottom layers and the bilayer to core-shell structural transition at B 74 where core-shell species start to dominate in thermody-namics, defining the up-limit of the bilayer boron nanoclusters at B 72 . The newly obtained bilayer C 2 B 64 (I), D 2 B 68 (III), and C 1 B 70 (IV) appear to be systematically more stable than the previously reported cage-like D 4d B 64 , core-shell C 1 B 68 , and quasi-planar C 3v B 70 , respectively. Detailed bonding analyses indicate that these bilayer species follow the universal bonding pattern of σ + π double delocalization, rendering three-dimensional aromaticity to the systems. The IR, Raman, and UV-vis spectra of the concerned bilayer species are computationally simulated to facilitate their future characterizations.