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Topic: Can you explain why the VSEPR model seems to fail here?  (Read 2975 times)

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Offline Fzang

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Can you explain why the VSEPR model seems to fail here?
« on: June 21, 2011, 03:12:07 PM »
Most geometries seem easy to explain with the VSEPR model, however this one had me wondering:


Why is the top example the 'accepted' geometry, while the bottom one appears to be more favorable, when the lonepairs are furthest apart?

Offline BluePill

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Re: Can you explain why the VSEPR model seems to fail here?
« Reply #1 on: June 21, 2011, 03:44:01 PM »
I think this would be easier if we rank how important the electron interactions are in bonding: lone pair - lone pair (lp-lp) >> lone pair - bond pair (lp-bp), and bond pair-bond pair (bp-bp). The larger the separation between electrons the better, right?

Let's inspect your answer:

lp-lp interactions: 180o [1]
lp-bp interactions: 90o [6]
bp-bp interactions: 120o [3]

lp-lp interaction is good but you also have to consider the lp-bp interactions. There are 6 and most importantly they are 90o

For the correct answer:

lp-lp interactions: 120o
lp-bp interactions: 90o [4] + 120o [2]
bp-bp interactions: 90o [2]

Both structures would have large angle between lp-lp. So the decision is between lp-bp and bp-bp. Looking at the number of lp-bp interactions at 90o for your answer and the correct answer, the latter would have less 90o interactions than your answer. Less repulsions and therefore, much preferred.

Offline Fzang

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Re: Can you explain why the VSEPR model seems to fail here?
« Reply #2 on: June 21, 2011, 03:54:20 PM »
So the bottom one has, on average, more repulsing interactions than the top one?

Thanks a lot, I hadn't considered that at all :)

Offline BluePill

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Re: Can you explain why the VSEPR model seems to fail here?
« Reply #3 on: June 21, 2011, 03:55:40 PM »
Yep. Don't worry I was stuck with that question before till I did some digging on VSEPR theory.

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