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
Transition metal alkyne complex acetylene during the production of ethylene. It is also useful in the preparation of fine chemicals. Semihydrogenation affords cis alkenes. Metal-alkyne complexes are also intermediates in the metal-catalyzed trimerization and tetramerizations. Cyclooctatetraene is produced from acetylene via the intermediacy of metal alkyne complexes. Variant of this reaction are exploited for the synthesis of substituted pyridines. The Pauson-Khand reaction provides a route to cyclopentenones via the intermediacy of cobalt-alkyne complexes. With the shift away from coal-based (acetylene) to petroleum-based feedstocks (olefins), catalytic reactions with alkynes are not widely practiced industrially. Acrylic acid was once prepared by the hydrocarboxylation of acetylene: Transition