The reactions of the third transition series metal hexafluorides, MF6, M = Pt, Ir, Os, Re and W, with carbon monoxide in superacid media

The reactions of the third transition (5d) series metal hexafluorides PtF6, IrF6, OsF6, ReF6 and WF6 with CO are studied in the Lewis superacid SbF5, the Brønsted superacid HF and the conjugate Brønsted–Lewis superacid HF–SbF5, in order to evaluate their potential as reactants in reductive carbonylations. These reactions are aimed at the generation of homoleptic metal carbonyl cations of the type [M(CO)n]m+, M = Pt, Ir, Os or Re, n = 4 or 6 and m = 1, 2 or 3. Under very mild conditions, PtF6 is cleanly converted in liquid SbF5 to give [Pt(CO)4][Sb2F11]2 in quantitative yield. In anhydrous hydrogen fluoride, PtF6 is reduced and partly carbonylated to the mixed valency product [Pt(CO)4][PtF6], the first carbonyl fluoride of platinum. With IrF6 in SbF5, [Ir(CO)6][Sb2F11]3 is obtained, which contains with [Ir(CO)6]3+ the first and so far only known homoleptic tripositive metal carbonyl cation. The reaction of OsF6 with CO in HF–SbF5 produces single crystals of [Os(CO)6][Sb2F11]2, which are suitable for a molecular structure determination. ReF6 reacts in SbF5 very slowly with CO, and the previously known [Re(CO)6]+ is only obtained in low yield. Finally WF6 does not react with CO in any of the superacid media under comparable conditions. As by-product in the reduction of MF6, M = Pt, Ir and Os, only carbonyl fluoride, COF2, forms, which allows very facile product isolation. The observed order of reactivity: PtF6 > IrF6 > OsF6 > ReF6 > > WF6 follows a previously established order of oxidizing ability and electron affinity of the metal hexafluorides. The mixed valency compound [Pt(CO)4][PtF6] is studied by vibrational spectroscopy and is found to contain a square planar [Pt(CO)4]2+ cation and a [PtF6]2− anion with a trigonally distorted octahedral geometry of pointgroup D3d. A vibrational assignment of [Ir(CO)6]3+ is presented, aided by 13C substitution and supported by ab initio calculations.