Interpreting Ring Currents from Hückel-Guided σ- and π-Electron Delocalization in Small Boron Rings.

The aromaticity of small boron clusters remains under scrutiny due to persistent inconsistencies between magnetic and electronic descriptors. Here, we reexamine B, B, B, B, and B using a multidimensional approach that integrates Adaptive Natural Density Partitioning, Electron Density of Delocalized Bonds, magnetically induced current density, and the z-component of the induced magnetic field. We introduce a model in which σ-aromaticity arises from two distinct delocalization topologies: a radial 2e σ-pathway and a tangential multicenter circuit formed by alternating filled and vacant sp orbitals. This framework accounts for the evolution of aromaticity upon oxidation or reduction, preserving coherence between electronic structure and magnetic response. B features cooperative radial and tangential σ-delocalization, together with a delocalized 2e π-bond, yielding robust double aromaticity. B retains σ- and π-aromaticity, but only via a tangential 6e σ-framework, leading to a more compact delocalization and slightly attenuated ring currents. In B, the presence of a radial 2e σ-bond and a 4c-2e π-bond confers partial aromatic character, while the tangential 8e σ-framework satisfies the 4n rule and induces a paratropic current. In contrast, B lacks the radial σ-component but retains a tangential 8e σ-circuit and a 2e 4c-2e π-bond, leading to a σ-antiaromatic and π-aromatic configuration. Finally, B, exhibits delocalized π- and σ-circuits, yielding consistent diatropic ring currents, which confirms its fully doubly aromatic nature. Altogether, this analysis underscores the importance of resolving σ-framework topology and demonstrates that, when radial and tangential contributions are correctly distinguished, Hückel's rule remains a powerful tool for interpreting aromaticity in small boron rings.