[Fzang]

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?

[BluePill]

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

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

For the correct answer:

lp-lp interactions: 120^o
lp-bp interactions: 90^o [4] + 120^o [2]
bp-bp interactions: 90^o [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 90^o for your answer and the correct answer, the latter would have less 90^o interactions than your answer. Less repulsions and therefore, much preferred.

[Fzang]

So the bottom one has, on average, more repulsing interactions than the top one?

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

[BluePill]

Yep. Don't worry I was stuck with that question before till I did some digging on VSEPR theory.