Why is Fe3+ unable to carry oxygen in methemoglobinanemia?

[gravenewworld]

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?

 

[Borek]

No idea, but I suppose you should look at the changes in electronic configuration of Fe - whatever is able to keep the oxygen, disappears when iron is oxidized.

--

 

[chemisttree]

It is likely that the much stronger lewis acid cannot reversibly bond to O2 but rather, reacts with it. All 8 positions of the octahedral complex are blocked. It is likely that the oxygen carried by the Fe+2 normal heme group reacts with oxygen to form the Fe+3 oxidized species.