Hexakis(carbonyl)iron(II) Undecafluorodiantimonate(V), [Fe(CO)6][Sb2F11]2, and -Hexafluoroantimonate(V), [Fe(CO)6][SbF6]2, Their Syntheses, and Spectroscopic and Structural Characterization by Single Crystal X-ray Diffraction and Normal Coordinate Analysis

Hexakis(carbonyl)iron(II) undecafluorodiantimonate(V), [Fe(CO)6][Sb2F11]2, is conveniently prepared by the oxidative carbonylation of Fe(CO)5 with XeF2 as external oxidizer in the conjugate Brønsted-Lewis superacid HF−SbF5. The colorless compound crystallizes from the reaction medium in high purity. The molecular structure is obtained by single-crystal X-ray diffraction. The cation is a regular octahedron, while the vertex-shared di-octahedral [Sb2F11]- anion is distorted from D 4 h symmetry by bending and rotational processes, due to significant interionic interactions, primarily of the F···C type. Washing of [Fe(CO)6][Sb2F11]2 with anhydrous HF results in an unusual elution of SbF5 and the quantitative conversion to hexakis(carbonyl)iron(II) hexafluoroantimonate(V) [Fe(CO)6][SbF6]2. The molecular structure of the salt shows octahedral ions with slight tetragonal distortions for the cation (elongation) and the anion (compression). Both salts are thermally stable up to 150 °C. The averaged bond distances and the vibrational wavenumbers of [Fe(CO)6]2+ are nearly identical in both compounds. The [Fe(CO)6]2+ cation, the first and so far only isolated and structurally characterized dipositive, superelectrophilic carbonyl cation formed by a 3d-metal, is further characterized by a normal coordinate analysis (NCA). The obtained force constants are compared to those of the isoelectronic molecule Cr(CO)6. Changes in π-back-bonding affect the F CO/CO and F CO/MC interaction force constants more strongly than the stretching force constants F CO and F MC. All 13 fundamentals of [Fe(CO)6]2+ are detected and assigned with the help of the data obtained from the normal coordinate analysis and density functional calculations published previously. The electronic ground state 1A1g of the [Fe(CO)6]2+ cation is established by magnetic susceptibility measurements of polycrystalline [Fe(CO)6][SbF6]2 and [Fe(CO)6][Sb2F11]2 between 2 and 300 K. The magnetic impurity formed during synthesis is identified as Fe[SbF6]2 which has iron(II) in high spin (5T2g) ground state. Consistent with a diamagnetic ground state are the single line in the 57Fe Mössbauer spectrum (i.s. = −0.003(8) mm s-1 relative to α-Fe), obtained on polycrystalline samples and the single sharp line in the 13C NMR spectrum in DF solution at 178 ppm with J (57Fe-13C) of 19.2 Hz.