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)A, much shorter than those of six-coordinate [ Fe(OEP)(Im)(CO)], although vc-o is 1944 - 1948 cm(-1). The sixcoordinate species [Fe(OEP)(CO)(2)] has also been studied. The competition for d-back-bonding of two CO ligands leads to Fe - C distance of 1.8558(10)A and v(C-O) being increased to 2021 cm(-1). The Mossbauer spectrum has a quadrupole splitting constant of 0 mm/s at 4.2 K, indicating high electronic symmetry.