[Darren]

Why does PCl₃ and PCl₅ exist but only PI₃ and PBr₃ exist? Why isnt there PBr₅ or PI₅?

What are the factors that determine the existence of compounds or molecules? Besides the general stability of the compound, what are the more specific factors involved?



Thank you in advance

[XGen]

This is just a wild guess, but perhaps something to do with the size of the atoms? Br and I are larger than Cl, so maybe less of them can fit around P?

[Sophia7X]

Yeah, Br and I are much larger than P, so several atoms of Br or I cannot be arranged in an energetically favorable manner around a small P atom. Also, the bond length of, for example, the phosphorus-iodine bond is too short. (Imagine trying to build a model of XI₅ with large Styrofoam balls around a small Styrofoam ball, using short toothpicks instead of long toothpicks. You can imagine it would be a lot harder, right?) There is such thing as PI₅, but it's ionic and it really is PI₄⁺I^-

[Darren]

Yeah, Br and I are much larger than P, so several atoms of Br or I cannot be arranged in an energetically favorable manner around a small P atom. Also, the bond length of, for example, the phosphorus-iodine bond is too short. (Imagine trying to build a model of XI₅ with large Styrofoam balls around a small Styrofoam ball, using short toothpicks instead of long toothpicks. You can imagine it would be a lot harder, right?) There is such thing as PI₅, but it's ionic and it really is PI₄⁺I^-

Ok thank you

[cheese (MSW)]

Then why doesn't PH5 exist?  Why does AsCl5 dec >-50°C? There is more to the story than a simple
sterics. 

[Darren]

Then why doesn't PH5 exist?  Why does AsCl5 dec >-50°C? There is more to the story than a simple
sterics. 

PH5 doesnt exist because there is too little electronegativity difference between the two elements? Im just making a guess. This may result in a lot less energy being released to form it, so its not energetically favourable and will be unstable.

[cheese (MSW)]

The P-H bond energy is ~322 kJ mol^-1 for P-Cl it is 326 kJ mol^-1.
http://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html
There is no evidence for PH5 even at low T.
Keep thinking!

[Sophia7X]

Then why doesn't PH5 exist?  Why does AsCl5 dec >-50°C? There is more to the story than a simple
sterics.  

I would think that it has something to do with the electron donating vs accepting nature. In PH5, the phosphorus would be more or less accepting 5 electrons from the less electronegative hydrogens. But in PCl5, the phosphorus would be donating 5 electrons to the chlorines.
Phosphorus should normally either accept 3 or give 5 electrons because it has 5 valence electrons.

However, not sure why AsCl5 is unstable while PCl5 is stable... maybe it has something to do with full 3d subshell?

[cheese (MSW)]

I admire the enthusiasm (and promise) the younger members show.
(Everyone is younger than me. )
You need a mentor to point you in the right direction!
Was this posted on the High School forum?  If so this explanation will probably
be over your head.
Consider the formation of PX5 (X = H, F, Cl, Br) from PX3
X3P:  +  X2  ⇋ PX5
-RTlnKeq = ΔG = ΔH - TΔS  Let us ignore the ΔS term even though there is loss of entropy in forming the product (why?). 
The ΔHrxn term may be broken down into three steps:
(1) Promotion energy: energy need to take a lone pair sp3 electron and place it in a 3d atomic orbital: 
P 3s23p4(P:F bonds)p(↑↓)(lp) → P 3s23p4p(↑)3d(↑)  ΔHprom +ve
We can also lump the reorganization energy into this term as well (change of F-P-F angles when forming the product)
and steric interactions.  Electron excitation energies are large and positive.
(2) Dissociation energy of X2:
X2(g) → 2X.    ΔHdiss +ve
(3) Energy of formation of two new P:X bonds = 2ΔHform(P-X) (-ve)
Hence ΔHrxn = ΔHprom +  ΔHdiss(X2)  + 2ΔHform(P-X) 
Take ΔHprom = 340 kJ mol^-1 (I know I am fixing the result!) and get ΔHdiss(X2)  and ΔHform(P-X)  from the table.
If ΔHrxn is –ve it is thermodynamically allowed, if +ve it is not allowed

X   ΔHdiss(X2)  kJ mol^-1   ΔHform(P-X) kJ mol^-1 
H   436                            322
F   155                            490
Cl   242                            326
Br   193                            264

Conclusion: the nonexistence of PBr5 could well be due steric factors,
but it is not the reason for the nonexistence of PH5. (What is the reason?)
P-Cl bond strength > As-Cl bond strength.
We can leave the discussion of why the P-X strengths vary the way they do to another day.
But it is a matter of overlap of the AOs.

[Darren]

Thanks cheese