Dear WW: To bad Alex, doc_comfort, and wofahullicodoc are not available to discuss this.
They know more about this than I ever did, and I have forgotten most of what I once knew.
But I doubt that the oxygen is binding directly to the iron. Oxygen has a valence of two,
and I'll bet all of the iron's valene is already involved. So while the iron is indispensible, it
just makes possible a huge molecule that can bind oxygen loosely enough
to give up in the tissues where it is needed, and then take up CO2 to take back to the lungs.
Hemoglobin is a very complicated molecule. And its size and complexity have more to do with
its color than the iron does. When hemoglobin is broken up, the iron is recycled, and the residue
give the orange-yellow-brown color to bile and stools. The color change is mostly due to smaller
molecule size.