Metal-Containing Borane and Hydrocarbon Sandwich Complexes: Exploring the 2D-3D Aromaticity Dilemma

Recent reported planar metalloborane architectures raise the question of how aromaticity should be classified when conventional 2D π-aromatic ligands and boron-based frameworks capable of 3D multicenter delocalization are incorporated into structurally analogous metal complexes. Specifically, it remains unclear whether aromatic delocalization in these systems is predominantly localized within the ligands or extends through the metal to generate a three-dimensional (3D) framework. In this work, we employ DFT and a suite of complementary aromaticity indicators to analyze a series of sandwich complexes, including the recently reported CpOsB6H11 and [Os(B5H10)2], in comparison with their hydrocarbon counterparts CpOsCp and ferrocene. For the systems investigated here, our results reveal markedly different degrees of metal integration into the aromatic framework. Coordination to the nido-borane frameworks produces enhanced global delocalization, greater metal-ligand multicenter delocalization, and more extensively coupled current-density pathways involving the metal. In contrast, the investigated cyclopentadienyl and related hydrocarbon complexes retain predominantly ligand-centered aromatic delocalization upon coordination, despite some degree of metal participation. On the basis of the combined magnetic, electron-delocalization, and orbital analyses, we propose a working classification ranging from predominantly ligand-centered 2D aromaticity to increasingly metal-integrated 3D aromaticity. These descriptions are not intended as rigid or universal categories, but rather identify the dominant aromatic-delocalization regime within the series examined.