σ-bonded metal carbonyl cations and their derivatives:: Syntheses and structural, spectroscopic, and bonding principles

Homoleptic sigma-bonded metal carbonyl cations (sigma-carbonyls) and their derivatives have grown in recent years into a large subgroup of mononuclear metal carbonyl complexes. They are at present formed by 17 elements, including the post transition metal mercury. Most are generated in superacids: the Lewis superacid SbF5 and the conjugate Bronsted-Lewis superacid HF-SbF5. Thermally stable salts are formed with the superacid anions [Sb2F11](-) and [SbF6](-). Many of the cations are superelectrophilic, with metals in oxidation states of +2 or +3. Three different synthetic routes are developed: reductive (A) or solvolytic carbonylations (B) and oxidative methods (C). Considerable progress has been made by replacing the Lewis superacid SbF5 as reaction medium by the conjugate Bronsted-Lewis superacid HF-SbF5. This allows reactions to proceed faster in a homogeneous phase and to produce crystalline materials. In total 27 mostly recent molecular structures have been obtained, including that of [Ir(CO)(6)] [SbF6](3)(.)4HF, the first HF solvate with one of the HF molecules coordinated in an isotridentate mode to three carbon atoms of the cation. Due to the electrophilic nature of the carbonyl carbon in fluoroantimonate salts of metal carbonyl cations, extended structures are formed via significant interionic C--- F contacts. A new a-bonded carbonyl, (CF3)(3)BCO, which has been fully characterized, expands the existence range of metal carbonyl cations to group 13. The large amount of spectroscopic and structural information accumulated is used to present a conceptually simple bonding model.