The Synthesis, Vibrational Spectra, and Molecular Structure of [Ir(CO)6][SbF6]3·4HF − The First Structurally Characterized Salt with a Tripositive, Homoleptic Metal Carbonyl Cation and the First Example of a Tetrahedral Hydrogen-Bonded (HF)4 Cluster

The reductive carbonylation of IrF6 in a dilute solution of SbF5 in anhydrous HF (1:6 by volume) produces surprisingly at 25 °C and 1.5 atm CO the complex salt [Ir(CO)6][SbF6]3·4HF, while [Ir(CO)6][Sb2F11]3 is obtained in liquid SbF5 under similar conditions. Vibrational spectra in the CO stretching range for both salts and [Ir(CO)6]3+ (solv) are identical within error limits, and ν(CO)av is with 2269 cm-1 the highest average stretching frequency so far observed for octahedral metal carbonyl cations. A vibrational assignment supported by DFT calculations is presented, and the vibrational fundamentals are compared to those of [Os(CO)6]2+. The molecular structure of [Ir(CO)6][SbF6]3·4HF is determined by single-crystal X-ray diffraction. Crystal data for [Ir(CO)6][SbF6]3·4HF: rhombohedral, R3c (No. 161), a = 14.630(4) Å, c = 18.377(7) Å, V = 3406.4(18) Å3, Z = 6, T = 150 K, R 1 = 0.0338 [I > 2σ (I)], wR 2 = 0.0797). The average Ir−C bond length in the octahedral [Ir(CO)6]3+ cation is with 2.029(10) the longest observed for iridium carbonyl derivatives, consistent with the absence of Ir → CO π-back-bonding. The four solvate HF molecules form a tetrahedron via long, asymmetric, and partly delocalized hydrogen bonds with F−F edge lengths of 2.857 (3x) and 2.914 (3x) Å. There is no precedent for a polyhedral (HF) n cluster in the gas, liquid, or solid phase. The four F atoms of the (HF)4 cluster are coordinated to the C atoms of the six CO ligands of the cation, which again is without precedent. The coordination of one of the F atoms to three C atoms in a iso-tridentate mode with contact distances C−F(8) of 2.641(10) Å is most unusual. The observed tight C−F coordination in [Ir(CO)6][SbF6]3·4HF provides conclusive evidence for the presence of electrophilic carbon in the cation and illustrates how superelectrophilic cations such as [Ir(CO)6]3+ are solvent stabilized in the conjugate Brønsted−Lewis superacid HF−SbF5.