As an electron-deficient element, boron possesses fascinating three-dimensional structures and unconventional chemical bonds. Nanoclusters of boron have also been found to exhibit intriguing structural properties, observed to have predominantly planar structures, in stark contrast to bulk boron allotropes, which are composed of the ubiquitous B ₁₂ icosahedral building blocks. Here, we report observation of the 2D-to-3D transition and bulk-like structural features in the size-selected boron clusters, as revealed by photoelectron spectroscopy, chemisorption experiments, and first-principles calculations. In the small to medium cluster size range, planar boron cluster anions are found to be unreactive and only B ₄₆ ^– and B ₅₆ ^– are observed to chemisorb C ₂ H ₄ and CO under ambient conditions, suggesting major structural transitions at these cluster sizes. Notably, B ₅₆ ^– is also found to be able to chemisorb and activate CO ₂ . The global minimum of B ₄₆ ^– is found to adopt a core-shell structure (B ₂ @B ₄₄ ^– ), consisting of a B ₂ core within a B ₄₄ shell, reminiscent of the interstitial B ₂ dumbbells in the high-pressure γ -B ₂₈ form of bulk boron. More remarkably, both the global minimum and the second most stable isomer of B ₅₆ ^– exhibit nest-like configurations, featuring the iconic B ₁₂ icosahedral core surrounded by a B ₄₄ half-shell (B ₁₂ @ h- B ₄₄ ^– ), signifying the onset of bulk-like structural characteristics in boron nanoclusters.

Inspired by the newly discovered sandwich D_5d [Os(η⁵-B₅H₁₀)₂] as a carbon-free analog of ferrocene [Fe(η⁵-C₅H₅)₂] and based on extensive global searches and first-principles theory calculations, we present herein the viable possibility of a staggered sandwich bis(η⁶-cycloborane)chromium D_6d [Cr(η⁶-B₆H₁₂)₂] (1) with two equivalent η⁶-B₆H₁₂ ligands isovalent with benzene as the boron analog of the eclipsed bis(η⁶-benzene)chromium D_6h [Cr(η⁶-C₆H₆)₂] (2). Detailed coordination bonding pattern, energy decomposition, and magnetically induced ring current analyses indicate that D_6d 1 matches the 18-electron rule (1S²1P⁶1D¹⁰) and exhibits similar aromaticity to D_6h 2, rendering high stability to the system. The metal–ring coordination bonding in D_6d 1 is dominated by effective Cr → (η⁶-B₆H₁₂)₂ δ-back-donations which appear to be significantly stronger than the corresponding Cr → (η⁶-C₆H₆)₂ δ-back-donations in D_6h 2, leading to an unusually short ring–ring distance of 2.70 Å and a large HOMO-LUMO gap of 7.06 eV in 1. D_6d 1 and D_6h 2 can also be used as building blocks to form their sandwich tetramers D_2h [Cr₄B₄₈H₇₆] (3) and D_2h [Cr₄C₄₈H₂₈] (4) and sandwich two-dimensional nanosheets [Cr(η⁶-B₆H₆)₂] (5) and [Cr(η⁶-C₆)₂] (6) via partial or complete dehydrogenations, respectively.

Inspired by the newly discovered sandwich D_5d [Os(η⁵-B₅H₁₀)₂] as a carbon-free analog of ferrocene [Fe(η⁵-C₅H₅)₂] (Science 392, (2026): 411) and based on extensive global searches and first-principles theory calculations, we present herein the viable possibility of a staggered sandwich bis(η⁶-cycloborane)chromium D_6d [Cr(η⁶-(BH)₆H₆)₂] (1) with two equivalent η⁶- (BH)₆H₆ ligands isovalent with benzene as the boron analog of the eclipsed bis(η6-benzene)chromium D_6h [Cr(η⁶-C₆H₆)₂] (2). Detailed coordination bonding pattern, energy decomposition, and magnetically induced ring current analyses indicate that D_6d 1 matches the 18-electron rule (1S²1P⁶1D¹⁰) and exhibits similar aromaticity with D_6h 2, rendering high stability to the system. The metal-ring coordination bonding in D_6d 1 is dominated by effective Cr → (η⁶-(BH)₆H₆)₂ δ-back-donations which appear to be significantly stronger than the corresponding Cr → (η⁶-C₆H₆)₂ δ-back-donations in D_6h 2, leading to an unusually short ring-ring distance of 2.70 Å and large HOMO-LUMO gap of 7.06 eV in 1. D_6d 1 and D_6h 2 can also be used as building blocks to form their sandwich tetramers D_2h [Cr₄(BH)₄₈H₂₈] (3) and D_2h [Cr₄C₄₈H₂₈] (4) and sandwich two-dimensional nanosheets [Cr(η⁶-(BH)₆)₂] (5) and [Cr(η⁶-C₆)₂] (6) via partial or complete dehydrogenations, respectively.