Quantitative Geometric Descriptions of the Belt Iron Atoms of the Iron−Molybdenum Cofactor of Nitrogenase and Synthetic Iron(II) Model Complexes

Six of the seven iron atoms in the iron−molybdenum cofactor of nitrogenase display an unusual geometry, which
is distorted from the tetrahedral geometry that is most common in iron−sulfur clusters. This distortion pulls the iron
along one C3 axis of the tetrahedron toward a trigonal pyramid. The trigonal pyramidal coordination geometry is
rare in four-coordinate transition metal complexes. In order to document this geometry in a systematic fashion in
iron(II) chemistry, we have synthesized a range of four-coordinate iron(II) complexes that vary from tetrahedral to
trigonal pyramidal. Continuous shape measures are used for a quantitative comparison of the stereochemistry of
the Fe atoms in the iron−molybdenum cofactor with those of the presently and previously reported model complexes,
as well as with those in polynuclear iron−sulfur compounds. This understanding of the iron coordination geometry
is expected to assist in the design of synthetic analogues for intermediates in the nitrogenase catalytic cycle.