A Borane Sandwich Analogue of Ferrocene

Sandwich complexes built from planar aromatic ligands are a central class of compounds in organometallic chemistry, yet a boron-ring analogue of ferrocene has remained elusive despite decades of work. Here, an extensive global exploration of the FeB₁₀H₂₀ potential energy surface identifies the homoleptic iron sandwich η⁵,η⁵-Fe(B₅H₁₀)₂ as the global minimum for this composition. This structure constitutes a fully inorganic boron-ring analogue of ferrocene. Iron coordination reverses the stability ordering of the borane framework by stabilizing borane rings over the preferred nido motifs, thereby enabling η⁵ coordination of two borane ligands. Bonding, magnetic response, and real-space analyses indicate that the electronic structure is dominated by ligand-centered delocalization, reorganized and stabilized upon Fe (II) coordination, and that this delocalization is shaped primarily by multicenter B─H─B bonding rather than direct B─B σ bonding. Ligand-redistribution energetics further indicate that Cp-mediated routes do not provide a thermodynamic driving force toward the homoleptic complex and, together with known temperature constraints for borane anions and borane-ring rearrangements, support a Cp-free, temporally decoupled capture─then─anneal strategy. Beyond identifying η⁵,η⁵-Fe(B₅H₁₀)₂ as a plausible synthetic target, these results show that transition-metal coordination can invert the stability preferences of electron-deficient boron frameworks and thereby enable aromatic borane ligands that are otherwise in accessible in isolation.