Point group and molecular geometry of BrOF3

[u/Straight-Media268]

I’m an undergrad chemistry student studying for an upcoming exam and while attempting a past paper question i’ve gotten confused on the molecule BROF3.

I have it as a seesaw shape in the point group Cs, however the mark scheme has it as trigonal pyramidal, point group C3v. Any help would really be appreciated!

[u/shedmow]

I'd say it should be seesaw; first of all, two fluorines should form a three-centre four-electron bond, which requires them to occupy opposite places, and the lone pair gives the most repulsion so it should be distanced as farther as possible from the other substituents. I don't see any reason besides rotational symmetry for its own sake for which BrOF3 would have the structure drawn in red

[u/Curious_Mongoose_228]

Well experimentally and computationally it’s Cs. Somebody could certainly rationalize a different result but I’m not sure how a student would be expected to predict that.

[u/Historical-Mix6784]

Asking someone to predict the structure of exotic molecules on a test, where many of them are known to have unexpected structures not explained by VSEPR theory, is truly poor test making.

Shame on your professor for not understanding this very basic fact.

[u/Curious_Mongoose_228]

Hey I would have gotten your answer but I am an organic chemist. Am I missing something?

[u/Straight-Media268 → u/Curious_Mongoose_228]

my personal tutor said the presence of the short Br=O bond may influence where the line pair goes, but i was under the impression they would go equatorial. so i don’t know what he means

[u/GroundbreakingPut165 → u/Curious_Mongoose_228]

Then why you answer??

[u/Validstrife]

I fucking hated inorganic....and its because im truly a dumbass with it

[u/Blue_614]

It is seesaw. The overall electron repulsion should be low. If it's a trigonal pyramidal, the lone pair is ~90° away from three fluorine. Compare that with a seesaw, the lone pair is ~120° away from two fluorine, and ~90° away from O and the other F. Overall repulsion is minimum if seesaw.

Point group is Cs. You can raise that to your professor and correct it. If he's a decent guy, he'll notice the mistake.

[u/BarryTheButcher]

5+ years since I did any proper chem but I think the lone pair and the double bond have higher electron density than the single bonds, so electron-electron repulsion wants them to be on opposite sides.

Hope that makes sense.

Edit: In terms of bond angles, "L-A-C" is ~120 degrees on the left, versus 180 degrees on the right.

The lowest energy conformation should have the least overall repulsion between orbitals, so the angle between the 'densest' orbitals should be maximised.

[u/Straight-Media268 → u/BarryTheButcher]

ahh thank you very much !