Homoleptic, (sigma)-Bonded Octahedral Superelectrophilic Metal Carbonyl Cations of Iron(II), Ruthenium(II), and Osmium(II). Part 2: Syntheses and Characterizations of [M(CO)6][BF4]2 (M = Fe, Ru, Os)

As the first examples of homoleptic, (sigma)-bonded superelectrophilic metal carbonyl cations with tetrafluoroborate [BF4]- as the counter anions three thermally stable salts of the composition [M(CO)6][BF4]2 (M = Fe, Ru, Os) have been synthesized and extensively characterized by thermochemical, structural, and spectroscopic methods. A common synthetic route, the oxidative carbonylation of either Fe(CO)5 (XeF2 as the oxidizer) or M3(CO)12 (M = Ru, Os) (F2 as the oxidizer) in the conjugate Br(phi)nsted-Lewis superacid HF/BF3, was employed. The thermal behavior of the three salts, studied by differential scanning calorimetry (DSC) and gas-phase IR spectroscopy of the decomposition products, has been compared to that of the corresponding [SbF6]- salts. The molecular structures of [M(CO)6][BF4]2 (M = Fe, Os) were obtained by single-crystal X-ray diffraction at 100 K. X-ray powder diffraction data for [M(CO)6][BF4]2 (M = Ru, Os) were obtained between 100 and 300 K in intervals of 50 K. All three salts are isostructural and crystallized in the tetragonal space group I4/m (No. 87). As for the corresponding [M(CO)6][SbF6]2 salts (M = Fe, Ru, Os), similar unit cell parameters and vibrational fundamentals were also found for the three [BF4]- compounds. For the structurally characterized salts [M(CO)6][BF4]2 (M = Fe, Os), very similar bond parameters for both cations and anions were found. Hence, the invariance of structural and spectroscopic properties of [M(CO)6]2+ cations (M = Fe, Ru, Os) extended from the fluoroantimonates [Sb2F11]- and [SbF6]- as counteranions also to [BF4]-.