Question

How does ferrous iron coordinate nitrogen in a porphyrin ring?

Answer 1

Nitrogen is coordinating iron; not the other way around. You should look up coordination compounds in general inorganic textbooks. [Fe(haem)(His)(O2)][Fe(haem)(His)(OX2)] is similar enough to [Fe(NH3)6]2+[Fe(NHX3)X6]X2+ 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.