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. And so this is what it will look like when it will be bound to that diatomic oxygen. So this is a diatomic oxygen that is bound to this metal atom, and it pulls away the electron density, decreasing the size of the metal atom. And now the metal atom is able to fit into the center of that protoporphine plane.