 The use of strong acids succeeded in preparing gold carbonyl cations such as [Au(CO)2]+, which is used as a catalyst for the carbonylation of olefins.The cationic platinum carbonyl complex [Pt(CO)4]2+ can be prepared by working in so-called superacids such as antimony pentafluoride.Although CO is considered generally as a ligand for low-valent metal ions, the tetravalent iron complex [Cp*2Fe]2+ (16-valence electron complex) quantitatively binds CO to give the diamagnetic Fe(IV)-carbonyl [Cp*2FeCO]2+ (18-valence electron complex).