Synthesis and Mössbauer Characterization of Octahedral Iron(II) Carbonyl Complexes FeI₂(CO)₃L and FeI₂(CO)₂L₂: Developing Models of the [Fe]-H₂ase Active Site

A series of mono- and disubstituted complexes, FeI2(CO)xL4−x, x = 2 or 3, is conveniently accessed from simple mixing of N-heterocyclic carbenes, phosphines, and aromatic amines with FeI2(CO)4, first reported by Hieber in 1928. The highly light sensitive complexes yield to crystallization and X-ray diffraction studies for six complexes showing them to be rudimentary structural models of the monoiron hydrogenase, [Fe]-H2ase or Hmd, active site in native (FeII(CO)2) or CO-inhibited (FeII(CO)3) states. Diatomic ligand (ν(CO)) vibrational and Mössbauer spectroscopies are related to those reported for the Hmd active site. The importance of a serial approach for relating such parameters in model compounds to low spin FeII in the diverse ligation of enzyme active sites is stressed.