Metal carbonyls react with reducing agents such as metallic sodium or sodium amalgam to give carbonylmetalate (or carbonylate) anions:
:Mn(CO) + 2 Na → 2 Na[Mn(CO)]
For iron pentacarbonyl, one obtains the tetracarbonylferrate with loss of CO:
:Fe(CO) + 2 Na → Na[Fe(CO)] + CO
Mercury can insert into the metal–metal bonds of some polynuclear metal carbonyls:
:Co(CO) + Hg → (CO)Co−Hg−Co(CO)