Point groups/ VSPER Watch

Super199
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Can someone help me with this question.

So SF4 should have a lone pair of electrons added to the diagram.

I've worked out the point group for a and b. I can't seem to workout the point group for C (If someone can explain or tell me what it is).

To workout ground state it's something to do with the lp-lp, lp-bp and bp-bp repulsion. The lower the repulsion the more stable. If someone doesn't mind just talking me through them it would be appreciated
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MexicanKeith
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(Original post by Super199)
Can someone help me with this question.

So SF4 should have a lone pair of electrons added to the diagram.

I've worked out the point group for a and b. I can't seem to workout the point group for C (If someone can explain or tell me what it is).

To workout ground state it's something to do with the lp-lp, lp-bp and bp-bp repulsion. The lower the repulsion the more stable. If someone doesn't mind just talking me through them it would be appreciated
point groups are
a) Td
b) C2v
c) C3v (check you can see where the relevant symmetry elements sit)

so we know we have 4 bonding pairs and one lone pair, so thats 5 total pairs of electrons.

You should know that the structure will therefore be based on a trigonal bipyramid.

the structures b and c are based on the trigonal bipyramid.

It worth simply remembering that in this situation, VSEPR predicts the seesaw structure (structure B)

You can maybe justify this to yourself by thinking that the lone pair is the most sterically demanding so it adopts a position where it is close to only 2 bonding pairs (ie at 90 degree)

In structure C the lone pair is at 90 degrees to three bonding pairs so thats less stable.

So yeah, the C2v structure is the one adopted here
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Super199
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(Original post by MexicanKeith)
point groups are
a) Td
b) C2v
c) C3v (check you can see where the relevant symmetry elements sit)

so we know we have 4 bonding pairs and one lone pair, so thats 5 total pairs of electrons.

You should know that the structure will therefore be based on a trigonal bipyramid.

the structures b and c are based on the trigonal bipyramid.

It worth simply remembering that in this situation, VSEPR predicts the seesaw structure (structure B)

You can maybe justify this to yourself by thinking that the lone pair is the most sterically demanding so it adopts a position where it is close to only 2 bonding pairs (ie at 90 degree)

In structure C the lone pair is at 90 degrees to three bonding pairs so thats less stable.

So yeah, the C2v structure is the one adopted here
Thanks for the reply

A few questions if you don't mind :
For c) where is the highest axis of rotation? Is it just upwards making the other 3 fluorines trig planar so c3?
When we look at lp-lp do we want them to be as far away as possible or is there a rule. I read somewhere that if they are 180 apart you don't consider them?

You mentioned that trigonal bipyramidal structure adopts see-saw structure is there any others that adopt a certain structure?
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MexicanKeith
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(Original post by Super199)
Thanks for the reply

A few questions if you don't mind :
For c) where is the highest axis of rotation? Is it just upwards making the other 3 fluorines trig planar so c3?
When we look at lp-lp do we want them to be as far away as possible or is there a rule. I read somewhere that if they are 180 apart you don't consider them?

You mentioned that trigonal bipyramidal structure adopts see-saw structure is there any others that adopt a certain structure?
for c) the principal axis is the C3 you describe

I always just try to make the lone pairs and bonding pairs all as far apart as possible.

look at the table here https://en.wikipedia.org/wiki/VSEPR_theory#AXE_method
E is a lone pair X is a bonded atom. it shows you the shape for however many lone pairs and bonding pairs you have.
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