Predominantly sigma-bonded metal carbonyl cations (sigma-carbonyls): new synthetic,structural, and bonding concepts in metal carbonyl chemistry

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.