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.