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I know methemoglobin's heme group has a change on its shape, what makes oxygen unable to coordinate, but... What causes that change? Does the electronic structure of $\ce{Fe(III)}$ matter or it's only a charge/radius conseqüence?

My solution is the following:

As $\ce{Fe(III)}$ is a hard Lewis acid, it needs hard Lewis bases, such as $\ce{H2O}$ and not $\ce{O2}$. Furthermore, the His $\ce{F8 }$ proximal's imidazole bond with $\ce{ Fe }$ becomes stronger, as it's relation charge/radius is bigger in $\ce{ Fe(III)}$ than in $\ce{ Fe(II)}$, which causes a torsion of the heme group. Is that right?

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