MB6. Effects of Geometry

In addition to the other factors we have seen, the geometry of a coordination complex can play a role in influencing the reduction potential of the metal ion.  Changes in bond distances and bond angles as well as overall shape of the coordination environment can all have subtle effects on the ability of the metal to accept an electron.

Changes in bond length can contribute to changes in the reduction potential.  Imagine a metal ion in a tetrahedral coordination environment.  Suppose the ligands are constrained in such a way that they can't all reach their optimum metal-donor bond distances.  Maybe the metal ion is part of a metalloprotein, and the amino acid residues that form its coordination environment have restricted movement within the protein chain, so that one of the donors can't quite make optimal contact with the metal.  What would be the result?

From the point of view of geometry, the tetrahedral geometry would become a distorted tetrahedron.  Its shape would more closely resemble a trigonal pyramid, with an apical donor a little farther away than the others.  Considering the fundamental role that a ligand plays in a coordination complex, this donor isn't doing its job.  It isn't donating electron density to the metal very effectively.  As a result, the metal may become slightly more electrophilic.  Its reduction potential may increase.

This situation has been proposed as one of the reasons for the enhanced reduction potential of some blue copper proteins.   A methionine ligand that is a little more distant than usual has been implicated as a cause of this increased drive for reduction.  The relatively weak interaction with this ligand destabilizes the metal ion in the oxidized state more than it does in the reduced state.

Problem MB6.1.

Researchers in Japan prepared a series of N-heterocyclic carbene (NHC) complexes of iron(I) (Tatsumi, Organometallics 2016, 35, 1368-1375).

a)  Show why the NHC ligand is stable despite the lack of an octet on carbon.

b)  These ligands form an iron complex, [(NHC)₄Fe][PF₆].  What is the oxidation state (or charge) of the iron?

c)  X-ray crystallography showed that when R = Et, the geometry is square planar.  The Fe-C distances are between 1.947 - 1.972 �. 

 With R = iPr, the geometry is also square planar, and the Fe-C distances were 1.992 - 2.003 �. 

Propose a reason for the different bond lengths.

d) Cyclic voltammetry showed  E = -1.4 V for Fe(II)/Fe(I) when R = iPr, but E = -1.7 V when R = Et. 

Propose a reason for the difference in reduction potential.

e)  When the complex is prepared in THF solvent using R = Mes, a three-coordinate, T-shaped complex results, with two NHC ligands trans to each other and a coordinated THF ligand. 

Draw the complex.

f)  Cyclic voltammetry with R = Me  showed  E = -1.0 V for Fe(II)/Fe(I). 

Propose a reason for the the difference in reduction potential compared to with R = Et.

g) With R = Me, X-ray crystallography showed that the complex [(NHC)₄Fe][PF₆] displayed a tetrahedral geometry, although the complexes with R = Et or iPr were square planar.

Explain why those geometries are a surprise, given the relative size of a methyl and an ethyl group, and the relative crowding in a tetrahedral vs. a square planar environment.

h)  With R = Et or iPr, the flat NHC ligands are all aligned perpendicular to the square plane.  Why does this arrangement make the square planar geometry the less crowded one?

i)  Electron paramagnetic resonance indicated three unpaired electrons in the case with R = Me, but only one unpaired electron with R = Et or iPr.  Draw the d orbital splitting diagram in each case, with electrons.

j)  Cyclic voltammetry with the Fe(II) complex showed a E = -1.9 V when R = Me, but E = -1.7 V when R = Et.

 Suggest a reason for the difference in reduction potential, based on the d orbital diagrams.

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (retired) with contributions from other authors as noted.  It is freely available for educational use.

Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. 

This material is based upon work supported by the National Science Foundation under Grant No. 1043566.

Navigation:

Back to Redox Metals in Biology

Back to Reactivity Index

Back to Structure & Reactivity