Predominantly sigma-bonded metal carbonyl cations (sigma-carbonyls) are conveniently generated in the Lewis superacid SbF(5) or the conjugate Brønsted-Lewis superacid HFbondSbF(5), primarily by solvolytic or reductive carbonylations. Thermally stable salts are formed with the fluoroantimonate(V) ions [SbF(6)](-) and [Sb(2)F(11)](-). 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 (d(10)), square planar (d(8)) or octahedral(d(6)). 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.
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Willner et al. (2003) studied this question.