The synthesis of ionic carbonyl complexes is possible by oxidation or reduction of the neutral complexes. Anionic metal carbonylates can be obtained for example by reduction of dinuclear complexes with sodium. A familiar example is the sodium salt of iron tetracarbonylate (NaFe(CO), Collmans reagent'), which is used in organic synthesis.
The cationic hexacarbonyl salts of manganese, technetium and rhenium can be prepared from the carbonyl halides under carbon monoxide pressure by reaction with a Lewis acid.
:Mn(CO)Cl + AlCl + CO → [][]
The use of strong acids succeeded in preparing gold carbonyl cations such as [Au(CO)], which is used as a catalyst for the carbonylation of alkenes. The cationic platinum carbonyl complex [Pt(CO)] can be prepared by working in so-called superacids such as antimony pentafluoride. Although CO is considered generally as a ligand for low-valent metal ions, the tetravalent iron complex [Cp*Fe] (16-valence electron complex) quantitatively binds CO to give the diamagnetic Fe(IV)-carbonyl [Cp*FeCO] (18-valence electron complex).