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In first semester biochemistry, my professor said that iron forms a coordination complex with nitrogens in the heme group.

I'm failing to understand what this even means since the charge on nitrogen does not change as a result of this bond. My organic chemistry textbook and biochemistry textbook do not even have a basic explanation of what is meant by this, and I can't find anything on the web.

How does iron form a bond with two of the nitrogens in the porphyrin ring, the histidine, and the oxygen?

enter image description here

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    $\begingroup$ Nitrogen is coordinating iron; not the other way around. You should look up coordination compounds in general inorganic textbooks. $\ce{[Fe(haem)(His)(O2)]}$ is similar enough to $\ce{[Fe(NH3)6]^2+}$ as a starting point. I’m not sure if this site is the best place to answer the question. By the way, the porphyrinic nitrogens (two of the four) do have a charge associated with them; they are rendered in the image as forming a single bond to iron. $\endgroup$
    – Jan
    Feb 12, 2017 at 20:00

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This is something I have asked myself for a long time until I finally realized by myself. I have superficial knowledge on this type of chemistry but I believe this will be enough, so i will explain it in a very simple fashion. Well you have to look at Nitrogen first, nitrogen doesn’t change it’s charge just like you said, which seems weird at first since it forms four bonds. Now think of nitrogen in the lewis formula, what does it look like? Nitrogen has a free pair of electrons available. Think of the d-orbitals in Fe, their character and shape. Think of metals in general, having their electrons in a type of diffuse cloud. Now to the bond, the Iron is capable of grabbing these free electron pairs and use them in a bond, pulling them into it’s direction. This way the charge doesn’t change. The nitrogen “donates” it’s electrons which do not participate in regular bonding like you would see it in ammonia, but in complex bonds. Hope this is enough to clarify :) Greetings from Germany

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This one I think I remember. There are two nitrogens atoms that act as an unstable free radical and take more two eletrons of iron. The iron atom now has four posiitive charges. Those four positive charges naturally atracts all negative charges around, the pairs of the three nitrogens and the pair of the oxygen, Iron would stay with four negative charges, if wasn't the fact that the donators atoms also stays with four positive charges, resulting in zero.
Exactly as in the positive ion of ionized water, the total charge is conserved. In water, it is conserved as one positive charge, in heme molecule with iron 2, the total charge is zero. It's not two because of the 2 eletrons of the two nitrogens tha entered the system.

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    $\begingroup$ Except porphyrin isn't a radical with unpaired valence electrons. $\endgroup$
    – Buttonwood
    Jul 11, 2021 at 14:52

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