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Topic: VSPER structures of Two compounds  (Read 3985 times)

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Offline Enantiomer

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VSPER structures of Two compounds
« on: October 07, 2009, 01:51:18 PM »
Hi, I'm going over a practice test and I found a question asking for VSPER structures, which is generally easy enough but I found a bit of a road block, (I think):

PF4-- Which I believe to have seesaw structure, (4 BP/1LP)
This is the one that has me mildly unsure
PF4+-  Which I'm guessing would form a tetrahedral?  It's hard for me to tell if it changes shape, is this wrong or right?  And if even if it does change shape is this due to a lesser repulsion force from the loss of the lone pair?
Thanks for your time

Offline cth

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Re: VSPER structures of Two compounds
« Reply #1 on: October 07, 2009, 02:56:33 PM »
PF4-- Which I believe to have seesaw structure
It is simpler than that: the P atom is in the centre and fluorine atoms around. But it should be distorted.

PF4+-  Which I'm guessing would form a tetrahedral?
Yes. It isn't distorted.

Why is that?
* First, think of PF3: there are 3 fluorine atoms and one lone pair around the P atom.
* Now, add F+ cation to PF3 to form PF4+. The fluorine atom has lost one electron to form the ion F+, so it has 6 valence electrons left: 3 lone pairs and one empty 2p orbital (which previously contained the electron that had been kicked off). What do you think the F+ cation will do when it meet with PF3? It will take over the lone pair located on the P atom to form a covalent bond (so called dative bond). So, in PF4+, the P atom has 4 fluorines around and no lone pair anymore :rarrow: it is tetrahedral.
* Now, add F- anion to PF3 to form PF4-. The fluorine atom has gained one electron to form F-, so it has 8 valence electrons. Its octet is filled. What do you think would happen when it meets with PF3? The fluorine won't take over the lone pair on the P atom. Instead, it is the P atom that will accept one lone pair coming from F- and put it in one of its 3d orbitals that are empty. So, the P atom will have 4 fluorine atoms around and will still have its own lone pair  :rarrow: 5 "objects" around the P atom, it won't be tetrahedral.

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