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 Bronsted superacid HF and the conjugate Bronsted-Lewis superacid HF-SbF 5, in order to evaluate their potential as reactants in reductive carbonyla tions. These reactions are aimed at the generation of homoleptic metal carb onyl cations of the type [M(CO)(n)](m+), M = Pt, Ir, Os or Re, n = 4 or 6 a nd 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 anhydr ous 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 wit h [Ir(CO)(6)](3+) the first and so far only known homoleptic tripositive me tal carbonyl cation. The reaction of OsF6 with CO in HF-SbF5 produces singl e crystals of [Os(CO)(6)][Sb2F11](2), which are suitable for a molecular st ructure determination. ReF6 reacts in SbF5 very slowly with CO, and the pre viously known [Re(CO)(6)](+) is only obtained in low yield. Finally WF6 doe s not react with CO in any of the superacid media under comparable conditio ns. As by-product in the reduction of MF6, M = Pt, Lr and Os, only carbonyl fluoride, COF2, forms, which allows very facile product isolation. The obs erved order of reactivity: PtF6 > IrF6 > OsF6 > ReF6 > > WF6 follows a prev iously established order of oxidizing ability and electron affinity of the metal hexafluorides. The mixed valency compound [Pt(CO)(4)][PrF6] is studie d by vibrational spectroscopy and is found to contain a square planar [Pt(C O)(4)](2+) cation and a [PtF6](2-) anion with a trigonally distorted octahe dral geometry of pointgroup D-3d. A vibrational assignment of [Ir(CO)(6)](3 +) is presented, aided by C-13 substitution and supported by ab initio calc ulations. (C) 2000 Elsevier Science S.A. All rights reserved.