Heme configuration

[Little Etoile]

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.Gchem book passage 25, #4:

Question asks for the configuration of the coordinate covalent bonds around iron ion in heme. Book says it is square planar because "metal ions have no non-bonding e- 's". But wouldn’t that make it tetrahedral? Square planar has two lone pairs.
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As always, thanks in advance!

 

[sleepy425]

.Gchem book passage 25, #4:

Question asks for the configuration of the coordinate covalent bonds around iron ion in heme. Book says it is square planar because "metal ions have no non-bonding e- 's". But wouldn’t that make it tetrahedral? Square planar has two lone pairs.
.

As always, thanks in advance!

You're right, square planar normally has two lone pairs. but not always. the geometry also depends on what the ligand is. in this case, the ligand of the iron is a porphyrin, which is tetracoordinate (four coordination sites) and aromatic. This means that the four nitrogens that coordinate the metal cannot get into any configuration other than square planar. tetrahedral would require the plane to be broken.