Homoleptic Metal Carbonyl Cations of the Electron‐Rich Metals: Their Generation in Superacid Media Together with Their Spectroscopic and Structural Characterization

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Abstract
Homoleptic carbonyl cations of the electron‐rich metals in Groups 8 through 12 are the newest members of the large family of transition metal carbonyls. They can be distinguished from typical metal carbonyl complexes in several respects. Their synthesis entails carbonylation of metal salts in such superacids as fluorosulfuric acid and “magic acid” HSO3FSbF5. Thermally stable salts with [Sb2F11]− as counterion are obtained with antimony pentafluoride as reaction medium. Both the [Sb2F11]− anion and superacid reaction media have previously found little application in the organometallic chemistry of d‐block elements. Also unprecedented in metal carbonyl chemistry are the coordination geometries with coordination numbers 4 (square‐planar coordination) and 2 (linear coordination) for the cation. Formal oxidation states of the metals, and the charges of the complex cations, extend from + 1 to +3: thus CO is largely σ‐bonded to the metal, and the CO bond is strongly polarized. Minimal metal → CO π‐backbonding and a positive partial charge on carbon are manifested in long MC bonds, short CO bonds, high frequencies for CO stretching vibrations (up to 2300 cm−1), and small 13C NMR chemical shifts (up to δc, = 121). Prominent examples of these unusual homoleptic carbonyl cations, which were recently the subject of a Highlight in this journal, include the first carbonyl cation of a p‐block metal [Hg(CO)2]2+, the first trivalent carbonyl cation [Ir(CO)6]3+, and the first multiply charged carbonyl cation of a 3d metal [Fe(CO)6]2+. In this overview we propose to (a) outline the historical origins of cationic metal carbonyls and their methods of synthesis; (b) present a summary of the general field of carbonyl cations, which has developed over a yery short period of time; (c) discuss the structural and spectroscopic characteritics of metal–CO bonding; (d) discuss the special significance associated with reaction media and the [Sb2F11]− anion; and (e) point to the most recent results and anticipated future developments.

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