Revisiting the Fe(0) Sandwich Complex [Fe(C6Me6)2], Structural Solution to a 64-Year-Old Mystery

In this communication, we revisit one of E. O. Fischer’s seminal discoveries to solve a textbook question: is the iron(0) complex [Fe(C₆Me₆)₂] an 18-valence-electron complex with a ‘ring-slip’ (η⁶,η⁴) configuration like its heavier congener [Ru(C₆Me₆)₂] or a 20-valence-electron complex with an (η⁶,η⁶) configuration like its neighbor [Co(C₆Me₆)₂]⁺? Using modern single-crystal X-ray diffraction techniques intertwined with density functional theory, we present the first crystallographically characterized iron(0) bis(arene) complex, [Fe(C₆Me₆)₂], demonstrating that iron opts to violate the 18-electron rule, favouring two aromatic arene ligands.