trans-[Fe(CN)4 (CO)2 ]2- , a 21st Century [Fe(CN)(CO)] Compound.

In 1887, more than a century after the synthesis of [FeII (CN)6 ]4- , the complex [FeII (CN)5 (CO)]3- , in which a CN- ligand is replaced by CO, was reported. It has taken an additional century for the synthesis of the complex in which a second CN- ligand is substituted by CO: trans-[FeII (CN)4 (CO)2 ]2- (structure shown). Remarkably, this anion is prepared by the simple reaction of FeCl2 with NaCN and CO in water. [Fe(CN)x (CO)y ] complexes are of interest as models for the active site of hydrogenase enzymes.