text: *Group 3 elements scandium and yttrium form monoanions, [M(CO)] (M = Sc, Y) which are 20-electron carbonyls, as does the lanthanide lanthanum.
  *Group 4 elements as dianions resemble neutral group 6 derivatives: [Ti(CO)].
  *Group 5 elements as monoanions resemble again neutral group 6 derivatives: [V(CO)].
  *Group 7 elements as monoanions resemble neutral group 8 derivatives: [M(CO)] (M = Mn, Tc, Re).
  *Group 8 elements as dianaions resemble neutral group 10 derivatives: [[disodium tetracarbonylferrate|[M(CO)] (M = Fe, Ru, Os)]]. Condensed derivatives are also known.
  *Group 9 elements as monoanions resemble neutral group 10 metal carbonyl. [Co(CO)] is the best studied member.
  Large anionic clusters of nickel, palladium, and platinum are also well known.  Many metal carbonyl anions can be protonated to give metal carbonyl hydrides.