Two types of highly electrophilic cations, bare M 2 + or carbonyl cations, formed by the noble metals palladium and platinum (Group 10) and silver and gold (Group 11) in low oxidation states (+1 or +2) are generated in highly acidic media. The acidic media consists of: the Br nsted superacid HSO 3 F; the Lewis superacid SbF 5 ; and the conjugate superacid HSO 3 F-SbF 5 . Derived from these superacids are the very weakly nucleophilic anions SO 3 F - or Sb 2 F 1 1 - as counterions to stabilize the cations in solids. Finally, stabilization of the highly electrophilic cations by coordination to CO leads to isolable complexes, unprecedented in transition-metal chemistry in their structural, spectroscopic and bonding characteristics. The cations discussed include, in addition to bare M 2 + (M = Ag or Au): linear [M(CO) n ] + , M = Ag or Au, n = 1 or 2; cyclic [Pd 2 (μ-CO) 2 ] 2 + ; squareplanar [M(CO) 4 ] 2 + ; the fluorosulfate derivatives cis-M(CO) 2 (SO 3 F) 2 , M = Pd or Pt; and Ir(CO) 3 (SO 3 F) 3 . Recently, the first post-transition-metal carbonyl cations, [Hg n (CO) 2 ] 2 + , n = 1 or 2, have been obtained, stabilized by Sb 2 F 1 1 - . Characterization of these species by magnetochemical methods, vibrational and 1 3 C NMR spectroscopy, and single-crystal X-ray diffraction is discussed.