The top portion of that fe, that is exactly where that bond will be formed between the diatomic oxygen and that metal atom. Remember, it's this metal iron atom of the hemegroup that binds directly and holds onto that diatomic oxygen. So if this is the unbound state of our protoporphy, then when the oxygen actually binds, what happens is this diatomic oxygen moves from the top position of that fe, and it begins to pull away some of that electron density from that metal atom. Remember, oxygen is the second most electronegative atom on the periodic table, and it is much more electronegative than this metal iron atom. And so what that means is the electron density will be pulled away from that metal atom, decreasing the radius and the size of that metal atom. And so what happens is, because the size of this iron atom decreases, it now is able to fit at the center of that protoporphyrin group.