Predominantly σ‐Bonded Metal Carbonyl Cations (σ‐Carbonyls): New Synthetic, Structural, and Bonding Concepts in Metal Carbonyl Chemistry

AbstractPredominantly 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.