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How do you calculate the charge of the Iron in the heme complex? I know it should be 2+, but don't the nitrogens that bind to the iron have an overall charge of 2+ making the iron 2-? I know that does not make sense since iron is a cation, but based on the math isn't that what the formal charge calculates to be for the nitrogens?
In the marshmallow review session they said a way to find the oxidation state was to give the bonding electrons back to the ligand to find the formal charge, then use the formal charge of the ligands to find the oxidation state of the metal. So the nitrogens with the single bonds and lone pair will have a charge of -1, resulting in an overall charge of 2- for the porphyrin ligand.
To figure out the ligand's charge, give back the lone pair that it donated to the cation. This would mean that two of the nitrogens have a formal charge of 0 (1 lone pair, 1 double bond, and 1 single bond) while the other two nitrogens will have a formal charge of -1 (2 lone pairs and 2 single bonds). Since the molecule is neutral, the Fe would have a charge of +2 to cancel out the -2 charge of the nitrogens. This was explained at the final review session.
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