Chemical Forums

Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Fzang on June 21, 2011, 03:12:07 PM

Title: Can you explain why the VSEPR model seems to fail here?
Post by: Fzang on June 21, 2011, 03:12:07 PM
Most geometries seem easy to explain with the VSEPR model, however this one had me wondering:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Ff.cl.ly%2Fitems%2F0M2k0x1r2o1S002h1544%2Fnote.png&hash=6dd2171f6156ceac8d28485d7ccdc707d0306afd)

Why is the top example the 'accepted' geometry, while the bottom one appears to be more favorable, when the lonepairs are furthest apart?
Title: Re: Can you explain why the VSEPR model seems to fail here?
Post by: BluePill 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.
Title: Re: Can you explain why the VSEPR model seems to fail here?
Post by: Fzang 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 :)
Title: Re: Can you explain why the VSEPR model seems to fail here?
Post by: BluePill 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.