Interaction of η²-C₂B₁₀H₁₀ and Its Derivatives with Group-X Transition Metals: A Theoretical Study Using DFT

Carboranes, a unique class of icosahedral boron-carbon clusters, exhibit remarkable thermal stability, hydrophobicity, and bioinertness, making them attractive in diverse applications from drug delivery to organometallic chemistry. Their dehydrogenated derivatives, o-carborenes, serve as versatile ligands, binding transition metals in η¹ or η² modes, giving rise to well-defined carboranyl/η²-carborene complexes across early and late metals. While transition-metal-ligand interactions are traditionally described by the Dewar-Chatt-Duncanson (DCD) model, deviations involving combined dative and electron-sharing (D + E) interactions have been reported for aryne and C₆₀ ligands. Motivated by these precedents, we investigate the bonding in Group X transition metal complexes, TM-(η²-1,2-C₂B₁₀H_10-nX_n) (TM = Ni, Pd, Pt; n = 0-2; X = I, Br, Ph), using NBO, QTAIM, ELF, and EDA-NOCV analyses. These studies reveal that, unlike alkyne/aryne analogues adhering to the DCD model, the o-carborene complexes adopt the D + E bonding paradigm, where TM fragments interact as a cationic fragment with an anionic o-carborene fragment, stabilized by dominant electrostatic contributions complemented by π-sharing. This work highlights the distinctive bonding adaptability of o-carborenes and establishes their role as electronically flexible ligands beyond the scope of classical models.