Configuration of Iron in Heme

[justadream]

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Although I guess the problem isn't absolutely necessary, I am referring to TPR SW Gen Chem Passage 26 (pg 308).


"The configuration of the coordinate covalent bonds around the iron ion in heme is"
A) tetrahedral
B) spherical
C) square planar
D) octahedral

Answer: square planar

The picture in the passage shows iron forming 4 bonds with heme. How do you get square planar from this?

Excerpt from the provided answer explanation: "Polyhydrins are flat and this one is a tetradentate (forms four coordinate covalent bonds). Since all metal ions have no nonbonding electrons, the only possible shape of the ion atoms must be square planar"

I thought that square planar requires having 6 electron-groups (and 2 lone pairs). Can someone clarify this?


EDIT: sorry posted this in the wrong forum. If a moderator could move, that would be great.

 

[Lunasly]

Someone might have to correct me on this, but it is to my understanding that Iron can form 6 bonds in total. Four of the bonds are bound to the heme prosthetic group while one more bond is formed between Fe(II) and a histidine reside (5th coordination complex). Furthermore, oxygen can come in and interact with iron, forming a 6th coordination complex.

If this is the correct interpretation, then I would assume that since that heme prosthetic group is planar (given all of the conjugate it is spy hybridized) that the iron would also be found in the same plane.

 

[Direction of Time]

The four nitrogens that form coordinate covalent bonds with Fe are part of the porphyrin ring. The porphyrin ring is aromatic and hence assumes a planar conformation (no free rotation around sp2-sp2 bonds). Normally, four bonds around a central atom with no free electrons assume a tetrahedral conformation, but in this case the porphyrin ring is restricted to a planar conformation, so the geometry around the Fe atom is square planar.

 

[justadream]

But doesn't square planar require there to be non-bonding electrons?

I'm referring to charts like this:

http://chemistry.ncssm.edu/labs/molgeom/vseprgeo.jpg

 

[Direction of Time]

If the elements/groups bonded to the central atom are free to move around in 3-space then yes, you need 2 pairs of non-bonding electrons + 4 bonds around the central atom for them to form a square planar shape. In the case of porphyrin they are not, so the bonds around the Fe atom are "stuck" in the square planar configuration (even though VSEPR theory says that "ideal" shape is tetrahedral under these conditions).

 

[Czarcasm]

Technically, it could go either way since they didn't specify whether they wanted the electron domain or geometry. Domain is Octahedral, but the molecular geometry is square planar.

 

[justadream]

Okay thanks for the input everyone!

To generalize further: How do I know when such situations occur (apart from just memorizing the scenarios)? I might be missing something from the passage but I don't know how I would have inferred this without knowing it before.

Also, once you conclude that something is "planar", how do you know it has to be "square planar"? Are there other types to consider? I'm asking if there's a systematic way to know (e.g., for geometry, just count the electron groups).

Czarcasm - how do you know the domain is octahedral (6 electron groups)?

 

[Czarcasm]

Okay thanks for the input everyone!

To generalize further: How do I know when such situations occur (apart from just memorizing the scenarios)? I might be missing something from the passage but I don't know how I would have inferred this without knowing it before.

Also, once you conclude that something is "planar", how do you know it has to be "square planar"? Are there other types to consider? I'm asking if there's a systematic way to know (e.g., for geometry, just count the electron groups).

Czarcasm - how do you know the domain is octahedral (6 electron groups)?

My mistake, reading that again, I see they're asking for the configuration of the coordinate covalent bonds around the iron. With that in mind, the only option is square planar. Had they instead asked for the geometry around the iron atom, then I suppose it would be subjective. As far as answering this, I took a biochem class, so I vaguely recall this info. But I also recall the passage explaining this in detail. So regardless if I seen it or not, you should be able to extrapolate info from the passage and be able to answer.