Metal carbonyl anions: From [Fe(CO)₄]^2- to [Hf(CO)₆]^2- and beyond

A chronological survey of the syntheses and characterizations of homoleptic mononuclear metal carbonyl anions is presented, from the first isolated example, [Fe(CO)₄]^2-, to the most recently reported species, [Hf(CO₆]^2-. These materials are of historical interest, since they are the first compounds to contain transition metals in formal negative oxidation states, but remain important reagents in all areas of chemical synthesis and often closely resemble electron-rich nonmetallic compounds in their ability to function as Lewis and Brønsted bases, nucleophiles, and/or reducing agents. Prospects for the syntheses of new carbonyl anions for the very early and late transition metals and the lanthanides and actinides are also discussed.