Why Do Resonance Structures for ClO- and ClO4- Differ in Formal Charges?

[MathewsMD]

Hi,

When drawing the resonance structures for ClO-, two exist. One where the O has a formal charge of -1 and there is then a single bond. The other structure is Cl with -1 for formal charge. Why is this second structure an equal resonance structure (why does the Cl have a negative formal charge) when O has the higher electronegativity?

Also, my next question stem from the previous one. In ClO4-, you have 4 resonance structures where the negative formal charge alternates between the 4 oxygen atoms, and the average bond order is 7/4. Why does a fifth resonance structure not exist where the chlorine has the negative formal charge, and makes a double bond with EACH oxygen atom?

It seems like Cl can have a negative formal charge in ClO- and not ClO4-, and I would like an explanation for that, if possible.

Thanks!

 

[DrDu]

This are good questions. As resonance structures aren't observable experimentally, you have to rely on calculations. If you look at these, there is in fact no double bond between Cl and O and O is either neutral or carries a negative formal charge.

 

[MathewsMD]

This are good questions. As resonance structures aren't observable experimentally, you have to rely on calculations. If you look at these, there is in fact no double bond between Cl and O and O is either neutral or carries a negative formal charge.

Ok. I understand the basics of resonance structures. I realize Lewis structures are used as visual aids, and are never in this form at any time, while the actual structure is only the hybrid. Just to confirm, you're saying the calculated average bond order is 1.5 for ClO- and 7/4 for ClO4-, and that is the reason the the other possible structures are not considered, right? Are there any other molecules like ClO- where the formal negative charge in the drawn Lewis structures goes to the less electronegative atom? How about for a positive formal charge on the more electronegative atom?

Also, do you mind shedding some light on how average bond order is actually calculated using a method besides Lewis structures?

 

[DrDu]

No, I am saying that bonding has I high degree of ionicity in these compounds and that there are certainly no covalent double bonds between Cl and O in these compounds. Bonding in higher main group compounds is not that simple. A classic is the following article, which you may obtain via your library:
http://onlinelibrary.wiley.com/doi/10.1002/anie.198402721/abstract

 

[LudusRex]

I would like to address the questions raised about the structures of ClO4- and ClO-. The discrepancy between the formal charges of Cl and O in the two structures of ClO- can be explained by the concept of electronegativity. While oxygen is indeed more electronegative than chlorine, the overall stability of a molecule is determined not only by the electronegativity of its atoms, but also by their arrangement and distribution of electrons. In this case, the second structure with a negative formal charge on Cl is a valid resonance structure because it allows for a more even distribution of electrons and a more stable molecule overall, despite the higher electronegativity of oxygen.

Moving on to the question about the existence of a fifth resonance structure for ClO4-, it is important to note that resonance structures are not always equally likely to contribute to the overall structure of a molecule. In the case of ClO4-, the four resonance structures with alternating negative formal charges on the oxygen atoms are more stable and contribute significantly to the overall structure. The fifth proposed structure, with a double bond between Cl and each oxygen atom, would result in a formal charge of +1 on Cl and a formal charge of -1 on each oxygen atom. This distribution of charges is less stable and therefore, this fifth structure is not considered a significant contributor to the overall structure of ClO4-.

In summary, the existence and significance of various resonance structures is determined by a combination of factors including electronegativity, electron distribution, and overall stability. While it may seem counterintuitive that a negative formal charge can exist on Cl in one molecule and not in another, it is important to consider the other contributing factors in determining the most stable and accurate representation of a molecule's structure.