Coordination of Diatomic Ligands to Heme: Simply CO

The synthesis and molecular structures of three iron(II) porphyrinates with only CO as the axial ligand(s) are reported. Two five-coordinate [Fe(OEP)(CO)] derivatives have Fe−C = 1.7077(13)
and 1.7140(10) Å, much shorter than those of six-coordinate [Fe(OEP)(Im)(CO)], although νC-O is 1944−1948 cm-1. The six-coordinate species [Fe(OEP)(CO)2] has also been studied. The
competition for π-back-bonding of two CO ligands leads to Fe−C
distance of 1.8558(10) Å and νC-O being increased to 2021 cm-1.
The Mössbauer spectrum has a quadrupole splitting constant of 0
mm/s at 4.2 K, indicating high electronic symmetry.