Structure of [PCl4]+

Hi,

So i've done this aaages ago and couldnt find much help on the internet. I was asked to draw a dot and cross diagram of $[PCl_4]^+$. Could any 1 help me with thatp lz?


Thanks.

Phosphorus is in group 5 = five valence electrons
Each chlorine (7 electrons) provides 1 electron to form a single bond = 4 electrons
But the species has a positive charge overall = -1 electron
---------------------------------------------------------

can you take it from there?

Oh yes there is a dative covalent bond isnt there?

Thanks.

You cannot dative coordinate to chlorine as it already has 7 electrons.
Chlorine cannot dative coordinate to phosphorus as that would not sort out it's outer shell of 7 electrons...

In brief, no.

Structure is square planar if I remeber correctly?

Right. So would the right structure have P in the middle, covalently bonding to 4 chlorines. This would leave one electron in phosphorus' outer shell. Right? Now because this electron is not paired, is that the reason why the whole specie is +ve ?

Thakns.

You remember incorrectly...

Well it has 8 electrons - 4 electron pairs... hmm tetrahedral?

This is worrying as I'm starting a chemistry course at uni in 2 weeks lol

I'm afraid your maths is a little shaky:


Read the post above...

Phosphorus = 5
Chlorines = 4
Positive charge subtract one electron
--------------------
total electrons = 8
--------------------

I get how it equals to +ve overall. However i'm still unclear about the unpaired electron.

Phosphorus has 5 electrons in its outer shell and 4 are bonded covalently with Cl. What happens to the 5th unpaired electron?

CHeers.

it is gone

it is an ex-electron

it is no longer there

vamoosh - disappeared

Interesting. Is it because it is a free radical and reacts with (almost) anything that comes around?

It is a positive ion

Free radicals are uncharged.

You need to do some basic housekeeping...

Phosphorus (at no 15) has 15 protons and 15 electrons so overall it has no charge.

Each chlorine (at no. = 17) has 17 protons and 17 electrons, so overall no change

If you add up all of the protons in (hypothetical) PCl₄ = 15 + 17 + 17 + 17 + 17 = 83+
If you add up all of the electrons in (hypothetical) PCl₄ = 15 + 17 + 17 + 17 + 17 = 83-

So (hypothetical) PCl₄ has a charge = +83 - 83 = 0

However PCl₄⁺ has 1 positive charge. It cannot have gained a proton so ....

.... it must have lost an electron.

The electron can't be lost from a chlorine (as this would leave the Cl with only 7 in the valence shell), so it must have been lost from the Phosphorus valence shell.

Summary
--------
Phosphorus has four pairs of e's in its valence shell = tetrahedral.

At Alevel standard, do we just assume that the lost electron just disappears randomly?

Thanks for bearing with me

It does not disappear randomly...

... in the same way as a sodium ion has a positive charge - it was lost at the moment of forming the ion.

Right! Thanks a lot for helping me through this mate. Really appreciate it.

Did anyone mention it was tetrahedral?

Structure is square planar if I remeber correctly?

The only way to arrange four pairs of electrons (regions of charge density) is tetrahedral.

Summary
--------
Phosphorus has four pairs of e's in its valence shell = tetrahedral.