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(LI) 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 Br phi nsted-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-4h symmetry by bending and rotational processes, due to significant interionic interactions, primarily of the FC 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 (comprsssion). Both salts are thermally stable up to 150 degrees 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 pi-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 (1)A(1g), 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 and 300 K. The magnetic impurity formed during synthesis is identified as Fe[SbF6](2) which has iron(II) in high spin (T-5(2g)) ground state. Consistent with a diamagnetic ground state are the single line in the Fe-57 Mossbauer spectrum (i.s. = -0.003(8) mm s(-1) relative to alpha-Fe), obtained on polycrystalline samples and the single sharp line in the C-13 NMR spectrum in DF solution at 178 ppm with J (Fe-57-C-13) of 19.2 Hz.