Synthesis and spectroscopic characterization of hexakis(carbonyl)iron(II) undecafluorodiantimonate(V), [Fe(CO)(6)][Sb2F11](2)

Oxidative carbonylation of Fe(CO)(5) in liquid SbF5 and a CO atmosphere, with either AsF5 or Cl-2 as external oxidizing agents, produces [Fe(CO)(6)][Sb2F11](2) in very high yield. The resulting off-white solid material is thermally stable (above 200 degrees 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(SO3F)(3), M=Ru or Os. The stretching force constant of 19.82x10(2) N m(-1) and <(nu)over bar (CO) (av)>, of 2215.5 cm(-1) suggest significantly reduced pi-back-bonding. The single line Fe-57 Mossbauer spectrum at delta=-0.001(6) mm s(-1) relative to alpha-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.