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 M-3(CO)(12) (M = Ru, Os) (F-2 as the oxidizer) in the conjugate Bronsted-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, Flu, 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](-).