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