What molecules bind to iron ions in the center of heme units of hemoglobin?

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The iron in the center of the heme group can coordinate one ligand at a time, and in hemoglobin the natural, physiological ligand is molecular oxygen (O2). When O2 encounters deoxygenated hemoglobin in the lungs, it binds to the ferrous iron (Fe2+), forming oxyhemoglobin; this binding is reversible, allowing O2 to be released where it’s needed in tissues. Carbon monoxide (CO) can also bind to the iron, but that’s not the normal binding situation and CO binds much more tightly, which is dangerous because it blocks O2 delivery. Carbon dioxide (CO2) doesn’t bind to the iron center at all; it interacts with amino groups on the globin chains instead. Nitrogen (N2) isn’t a physiological ligand for the heme iron. So, the molecule that binds to the iron center in hemoglobin under normal physiology is oxygen.