Metal carbonyl 18-electron complexes is sometimes catalysed by catalytic amounts of oxidants, via electron-transfer. Metal carbonyls react with reducing agents such as metallic sodium or sodium amalgam to give carbonylmetalate (or carbonylate) anions: For iron pentacarbonyl, one obtains the tetracarbonylferrate with loss of CO: Mercury can insert into the metal-metal bonds of some polynuclear metal carbonyls: The CO ligand is often susceptible to attack by nucleophiles. For example, trimethylamine oxide and bistrimethylsilylamide 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
Metal carbonyl hydride the catalyst is usually formed "in situ" in a reaction of a metal salt precursor with the syngas. The hydroformylation starts with the generation of a coordinatively unsaturated 16-electron metal carbonyl hydride complex like HCo(CO) or HRh(CO)(PPh) by dissociation of a ligand. Such complexes bind olefins in a first step via π-complexation. In a second step an alkyl complex is formed by insertion of the olefin into the metal-hydrogen bond, leading once again to a 16-electron species. This complex can bind another carbon monoxide, which can insert into the metal-carbon bond of the alkyl ligand to form an acyl complex.