Methemoglobinanemia is when the iron atom in the heme group of hemoglobin is oxidized to the +3 state. In this oxidation state, the heme group is unable to carry oxygen. Why is this? A ton of the sources I have come across explain the condition of methemoglobinanemia and simply say that the ferric ion can't carry O2, but don't explain why the ferric ion can not carry O2. I want to understand this on a molecular level. I already have an understanding of how the iron binds to O2 in the +2 oxidation state from the papers that Pauling has written. Can anyone please explain why O2 can not bind to the heme group when the iron atom is oxidized to the +3 state?