Reduction of CO₂ to CO Using Low-Coordinate Iron: Formation of a Four-Coordinate Iron Dicarbonyl Complex and a Bridging Carbonate Complex

A reduced diiron(I) complex reacts with CO2 to give two iron-containing products. One product has a carbonate bridge, which isomerizes rapidly at −70 °C and may be derived from an oxodiiron intermediate. The formation of this product releases free CO, which leads to a four-coordinate iron dicarbonyl complex. This product is the first crystallographically characterized example of a four-coordinate iron dicarbonyl species, a moiety that may be present in the active site of Hmd (“iron−sulfur cluster free”) hydrogenase.