Friedhelm Aubke

Predominantly σ‐bonded metal carbonyl cations (σ‐carbonyls) are conveniently generated in the Lewis superacid SbF₅ or the conjugate Brønsted–Lewis superacid HFSbF₅, primarily by solvolytic or reductive carbonylations. Thermally stable salts are formed with the fluoroantimonate(V) ions [SbF₆]⁻ and [Sb₂F₁₁]⁻. The salts are characterised by analytical, structural, spectroscopic and computational methods. Most homoleptic carbonyl cations have very regular geometries, comensurate with their d‐electron configurations: linear (d¹⁰), square planar (d⁸) or octahedral(d⁶). The cations with metals in oxidation states of +2 or +3 are termed “superelectrophilic”. Extended molecular structures form by significant interionic CF contacts with electrophilic carbon as acceptor. To account for all experimental observations, a conceptually simple synergetic bonding model is proposed. An outlook at anticipated future developments based on very recent results is provided.

The first carbonyl halogen cation in the condensed phase, the superelectrophilic [ClCO]⁺ ion, has been clearly and unambiguously established by its complete, fully assigned vibrational spectrum. The cation is obtained by the reaction of COCl₂ with SbF₅ at room temperature in the form of a suspension, in which it is stabilized by oligomeric fluoroantimonate ions.

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 δ_c, = 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.