Synthesis and Spectroscopic Characterization of Hexakis(carbonyl)iron(II) Undecafluorodiantimonate(V), [Fe(CO)6][Sb2F11]2

Oxidative carbonylation of Fe(CO) 5 in liquid SbF 5 and a CO atmosphere, with either AsF 5 or Cl 2 as external oxidizing agents, produces [Fe(CO) 6 ][Sb 2 F 11 ] 2 in very high yield. The resulting off-white solid material is thermally stable (above 200 °C). The homoleptic carbonyl cation [Fe(CO) 6 ] 2+, the first of its type formed by a 3d-metal, is isostructural with [Ru(CO) 6 ] 2+ and [Os(CO) 6 ] 2+ which are obtained by reductive carbonylation of M(SO 3 F) 3, M = Ru or Os. The stretching force constant of 19.82 × 10 2 N m - 1 and ν̄(CO) av of 2215.5 cm - 1 suggest significantly reduced π-back-bonding. The single line 57 Fe Mössbauer spectrum at δ = −0.001(6) mm s - 1 relative to α-Fe is consistent with a regular octahedral structure and a 1 A 1g groundstate for [Fe(CO) 6 ] 2+ . The possible existence of [Fe(CO) 6 ] 3+ formed by oxidation with Cl 2 is discussed.