How does ferrous iron coordinate nitrogen in a porphyrin ring?
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 :) 
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