1) Number of coordinated bonds? 4 or 2 ??
2) Is the metal in the core charged or neutral ?
(See attached pic.)
It is neutral because porphyrin was deprotonated and since then it has 2- charge. Iron has 2+ so overall it is 0. Iron forms 4 coordinate bonds with deprotonated porphyrin which is aromatic. 5th bond links with protein. 6th with molecule of oxygen.
I agree. Iron in heme is 2+ and it can form 6 coordination bonds (4 are present in porphyrine itself).
Is that mean that heme (or chlorophyll) has an overall charge= +2 ??
Why it cannot be two covalent bonds and two coordinated bonds?
It is neutral because porphyrin was deprotonated and since then it has 2- charge. Iron has 2+ so overall it is 0. Iron forms 4 coordinate bonds with deprotonated porphyrin which is aromatic. 5th bond links with protein. 6th with molecule of oxygen.
Thanks Mr. Lewczuk for your answer.
I always think that the metal M2+ has 2- charge due to the gain of 4 electrons from the 4 coordinated bonds; and the deprotonated macrocycle has 2+ charge because two N atoms loose their electrons (dashed bonds in the picture). hence the overall charge is 0. So agree with you that 2- and 2+ contribute to the neutrality of porphyrin but the 2- is assigned to the metal and the 2+ is assigned to the macrocycle. What do you think?
I think what you are describing is how to calculate formal charge for a covalent molecule where the electronegativity of the atoms are ignored. Formal charge is typically used to calculate oxidation state of the atom especially non-metals, but I don't think it is applicable for metal complexes.
The stability and chemistry of metal complexes arises from all the electrons used in the co-ordinate bonding so I don't think it is correct to consider that the metal centre takes in 1 electron from the co-ordinate bond only. Hope it helps.
Coordinate bonds don't influence the charge because they are formed by shared electrons. Similary to covalent bond. The difference is electrons come from only one atom. Charge of iron will change if You add/take electrons from the outer shell (N).
Thanks Mr. Yang and Mr. Lewczuk..that's what I want to know. So formal charges are not applicable in the case of coordinated bonds.
Is that correct ?? (attached pic.)
Iron has the positive charge and the porphyrin has negative. But it is not a cause of existence of coordination bonds. I think that "minus" on only two nitrogens is just a simplification because all four bonds are the same.
ًُSir, your draws are not complete. The metals inside the porphyrin ring have coordination numbers of 5 or 6, depending the specific protein and its function. There might be one or two His, sometimes Met, etc.
@Rafal Lewczuk
I agree, that minus on the two N is for simplification. the 4 coordinated bonds are the same due to the aromacity. Right?
Dr. Abu-Sbeih
what about synthetic metalloporphyrins as
Zinc-5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin fro example??
(attached pic.)
Looking at that structures one can think that there are two covalent and two coordination bonds. But there are four coordination bonds in the plane. More realistic structure of porphyrin should have marked delocalization instead of series of single and double bonds.
I think that synthetic compounds have the same type of bonding.
Of course this complex has a C.N. of 4. I was talking about heme.
Thanks all for your answer.
I am still wondering if the charges (in green, pic. above) are correct like that ?? I am waiting your confirmation.
It is acceptable to make it simple, but partial charge (called as "sigma minus") is placed on four nitrogens in inner circle.
Hi
Dear researchers, I come back to this valuable discussion to thank you first for your clear answers. that really helped me!
Second, I would like to ask again if we have a trivalent metal like Ni3+ instead of Zn2+ or Fe2+ then the overall charge of the macrocycle is +1 and not neutral as the case of divalent ions (given that the ion is neutralized by nitrogen dianion). Am I right ??
Rafał Lewczuk
Khaleel A Abu-Sbeih
Eugene Yang
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