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Why is AX3E2 not trigonal planar?

Posted: Wed Nov 28, 2018 6:11 pm
by Henri_de_Guzman_3L
To me trigonal planar for AX3E2 would make more sense, putting the lone pairs on axial positions. This way they are 180 degrees apart from each other. Since lone pairs have the strongest repulsion wouldn't this be optimal? Why is it T-shaped instead?

Re: Why is AX3E2 not trigonal planar?

Posted: Wed Nov 28, 2018 6:39 pm
by Rachana Jayaraman 1H
I think it actually would be minimal repulsion because if there are two equatorial lone pairs,they would interact with only two atoms at less than 90 degrees (the two axial atoms) and one atom at <120 degrees. If they were the two axial lone pairs, they would interact with all three equatorial atoms at less than 90 degrees, thus having a much more repulsive effect on the equatorial atoms.
I'm not sure if this is completely true though. Professor Lavelle said in lecture that for AX4E, the lone pair is in the equatorial plane because the more repulsive lone pair interacts with only 2 bonds at 90 degrees, so I am assuming that the same is true for AX3E2.

Re: Why is AX3E2 not trigonal planar?

Posted: Wed Nov 28, 2018 6:58 pm
by Sarah Fatkin 4I
I think that the lone pairs are further away from everything if they are in the t-shaped position -- if you had a trigonal planar molecule with two lone pairs at axial positions then it would look kinda like trigonal bipyramidal, but because lone pairs repel more than bonds, it wouldn't work out to have maximum space between every atom.