The CO ligand is often susceptible to attack by nucleophiles. For example, trimethylamine oxide and potassium bis(trimethylsilyl)amide convert CO ligands to CO and CN, respectively. In the "Hieber base reaction", hydroxide ion attacks the CO ligand to give a metallacarboxylic acid, followed by the release of carbon dioxide and the formation of metal hydrides or carbonylmetalates. A well-studied example of this nucleophilic addition is the conversion of iron pentacarbonyl to hydridoiron tetracarbonyl anion:
:Fe(CO) + NaOH → Na[Fe(CO)COH]
:Na[Fe(CO)COOH] + NaOH → Na[HFe(CO)] + NaHCO
Hydride reagents also attack CO ligands, especially in cationic metal complexes, to give the formyl derivative:
:[Re(CO)] + H → Re(CO)CHO
Organolithium reagents add with metal carbonyls to acylmetal carbonyl anions. O-Alkylation of these anions, such as with Meerwein salts, affords Fischer carbenes.