First C 4 bridged mixed-valence iron( II )–iron( III ) complex delocalized on the infrared timescale

The first 35-electron acetylide bridged diiron complex [{Fe(η5-C5Me5)(dppe)}2(µ-C4)]PF6, 3PF6[dppe = ethylenebis(diphenylphosphine)] and the 36- and 34-electron bis-iron(II) and bis-iron(III) homologous derivatives are synthesized and it is established that the FeII–FeIII complex is the first non-trapped mixed-valence compound with two half-sandwich monomeric units joined by a C4 bridge.