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) wit h XeF2 as external oxidizer in the conjugate Br phi nsted-Lewis superacid H F-SbF5. The colorless compound crystallizes from the reaction medium in hig h purity. The molecular structure is obtained by single-crystal X-ray diffr action. The cation is a regular octahedron, while the vertex-shared di-octa hedral [Sb2F11](-) anion is distorted from D-4h symmetry by bending and rot ational processes, due to significant interionic interactions, primarily of the FC type. Washing of [Fe(CO)(6)][Sb2F11](2) with anhydrous HF results i n an unusual elution of SbF5 and the quantitative conversion to hexakis(car bonyl)iron(II) hexafluoroantimonate;(V) [Fe(CO)(6)][SbF6](2) The molecular structure of the salt shows octahedral ions with slight tetragonal distorti ons 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 com pounds. The [Fe(CO)(6)](2+) cation, the first and so far only isolated and structurally characterized dipositive, superelectrophilic carbonyl cation f ormed by a 3d-metal, is further characterized by a normal coordinate analys is (NCA). The obtained force constants are compared to those of the isoelec tronic 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 detecte d and assigned with the help of the data obtained from the normal coordinat e analysis and density functional calculations published previously. The el ectronic ground state (1)A(1g), of the [Fe(CO)(6)](2+) cation is establishe d by magnetic susceptibility measurements of polycrystalline [Fe(CO)(6)][Sb F6](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 stat e 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 si ngle sharp line in the C-13 NMR spectrum in DF solution at 178 ppm with J ( Fe-57-C-13) of 19.2 Hz.