Why does the top-right nitrogen in heme have only 4 valence electrons?

[fomenkoa]

Hey everyone

I am confused as to the structure of a heme molecule. A picture of it is shown here:

http://en.wikipedia.org/wiki/Image:Heme.png

I understand the heme group is largely a porphyrin molecule. The 4 Nitrogens are coordinate-covalently bound to the central Iron atom. The top-left Nitrogen in the diagram checks out. It has 5 valence electrons (3 bonded to the two carbons and 2 in the coordinate bond)

However, my question is : what about the top right Nitrogen? It seems to only have 4 valence electrons.

Does this have to do with the resonance of the carbon rings and how the double bonds can move around? Any help would be appreciated!

Also, Where are the valence electrons of the Iron since it doesent contribute to the coordinate bonds? Are they just lone pairs then?

Anton

 

[fomenkoa]

Just to add, the purpose of this question is to understand the shape of the heme group molecule using bonding theory...I have to write about all th ebond angles and such...whew...so I guess I need to understand any resonant structures and lone pairs, etc...

 

[GCT]

Alright, so let's start off with the question of whether this is a transition metal complex or a covalent structure.

From here on you can rationalize the questions you have asked e.g. in transition metal complexes, which orbitals hold the valence electrons for iron?

Also, consider the formal charges, and yes you'll need to consider resonance, the electron and molecular geometries

 

[GCT]

even in transition metal complexes there are degrees of covalency, but predicting their geometries through valence bond theory simply will not work, in most cases.