Homoleptic, sigma-bonded octahedral [M(CO)(6)](2+) cations of iron(II), ruthenium(II), and osmium(II). Part 1: Syntheses, thermochemical and vibrational characterizations, and molecular structures as [Sb(2)F(11)](-) and [SbF(6)](-) salts. A comprehensive, comparative study.

Homoleptic octahedral, superelectrophilic sigma-bonded metal carbonyl cations of the type M(CO)(6) (M = Ru, Os) are generated in the Bronsted-Lewis conjugate superacid HF/SbF(5) by reductive carbonylation of M(SO(3)F)(3) (M = Ru, Os) or OsF(6). Thermally stable salts form with either Sb(2)F(11) or SbF(6) as anion, just as for the previously reported Fe(CO)(6) cation. The latter salts are generated by oxidative (XeF(2)) carbonylation of Fe(CO)(5) in HF/SbF(5). A rationale for the two diverging synthetic approaches is provided. The thermal stabilities of [M(CO)(6)]SbF(6) salts, studied by DSC, range from 180 degrees C for M = Fe to 350 degrees C for M = Os before decarbonylation occurs. The two triads [M(CO)(6)]SbF(6) and [M(CO)(6)]Sb(2)F(11) (M = Fe, Ru, Os) are extensively characterized by single-crystal X-ray diffraction and vibrational and (13)C NMR spectroscopy, aided by computational studies of the cations. The three [M(CO)(6)]SbF(6) salts (M = Fe, Ru, Os) crystallize in the tetragonal space group P4/mnc (No. 128), whereas the corresponding Sb(2)F(11) salts are monoclinic, crystallizing in space group P2(1)/n (No. 14). In both triads, the unit cell parameters are nearly invariant of the metal. Bond parameters for the anions SbF(6) and Sb(2)F(11) and their vibrational properties in the two triads are completely identical. In all six salts, the structural and vibrational properties of the M(CO)(6) cations (M = Fe, Ru, Os) are independent of the counteranion and for the most part independent of M and nearly identical. Interionic C...F contacts are similarly weak in all six salts. Metal dependency is noted only in the (13)C NMR spectra, in the skeletal M-C vibrations, and to a much smaller extent in some of the C-O stretching fundamentals (A(1g) and T(1u)). The findings reported here are unprecedented among metal carbonyl cations and their salts.