[shelanachium]

Consider CF4, SiF4, GeF4, all gases at room temperature, and the boiling-points increase as the central atom gets heavier and larger, as you'd expect.

This is pretty general - as in SF6, SeF6, TeF6; C(CH3)4, Si(CH3)4, Ge(CH3)4 etc.

But SiCl4 is lower-boiling than CCl4; and WF6 is lower-boiling than MoF6. Why do these buck the trend?

[AhBeng]

Two days ago, I posted this very same question on the "Inorganic Chem" forum, which suffered a similar fate to this thread of Shelanachium's. No one seems to know why (CCl4 has a higher m.p. and b.p. compared to SiCl4).

Having scoured the internet for the last couple of days (Google only just directed me to this thread here on the "Phy Chem" forum) , I've uncovered over a dozen instances of people having asked this very same (chemical mystery of a) question since time immemorial (ok, minor exaggeration there), to which it seems no one on Earth knows the answer.

What are we missing here?

[Mitch]

Obviously the questions aren't easy, or we would answer them readily. I'll give the answer for why WF₆  has a lower boiling point then MoF₆.

The general argument is that the d-orbitals of Tungsten are more diffuse and thus bond better with fluorine. A more covalent molecule is usually a more volatile one, and thus it has a lower boiling point. We can examine it further by looking at their bond dissociation energies, the more covalent the bond the higher the energy needed to dissociate that bond

Mo - F  107 kcal/mol
W  - F  120 kcal/mol

Hope this helps.

[AhBeng]

Thanks for your reply, Mitch.

Yes, your explanation certainly helps us understand why WF6 has a lower boiling point then MoF6.

As an aside, for the convenience of other visitors to this thread, I've reproduced the m.p. and b.p. of CCl4 vs SiCl4 here.

CCl4
Melting point -22.92 °C
Boiling point 76.72 °C

SiCl4
Melting point −68.74 °C
Boiling point 57.65 °C
 
( Source :
http://www.answers.com/topic/carbon-tetrachloride
http://www.answers.com/topic/silicon-tetrachloride )

Obviously the questions aren't easy, or we would answer them readily. I'll give the answer for why WF₆  has a lower boiling point then MoF₆.

The general argument is that the d-orbitals of Tungsten are more diffuse and thus bond better with fluorine. A more covalent molecule is usually a more volatile one, and thus it has a lower boiling point. We can examine it further by looking at their bond dissociation energies, the more covalent the bond the higher the energy needed to dissociate that bond

Mo - F  107 kcal/mol
W  - F  120 kcal/mol

Hope this helps.

[shelanachium]

Quote from: Mitch on October 17, 2008, 10:56:15 AM

Obviously the questions aren't easy, or we would answer them readily. I'll give the answer for why WF6  has a lower boiling point then MoF6.

The general argument is that the d-orbitals of Tungsten are more diffuse and thus bond better with fluorine. A more covalent molecule is usually a more volatile one, and thus it has a lower boiling point. We can examine it further by looking at their bond dissociation energies, the more covalent the bond the higher the energy needed to dissociate that bond[/b]
Mo - F  107 kcal/mol
W  - F  120 kcal/mol

Hope this helps.


 
However the Si-Cl bond is stronger than the C-Cl bond but surely it is not more covalent!

All known hexafluorides are volatile because however polar the bonds it is impossible to produce a polymeric or ionic lattice without excessively high co-ordination numbers on the central atom. The same applies to tetrachlorides of the smaller elements, and to all known tetroxides (Ru, Xe, Os).

That compounds with highly polar bonds can be volatile if co-ordination numbers are restricted is also shown by such compounds as lanthanide bis(trimethylsilyl)amides Ln{[(CH3)3Si]2N}3, where the large size of the anions restricts co-ordination numbers.

It used to be thought that free quarks might exist and bind to nuclei. Consider imaginary 'quafline' Qf in which an up quark, charge +2/3, has bound to a fluorine nucleus. This would be most stable as Qf(-!/3) which would have an extremely high electron binding energy and would be most reluctant to form covalent bonds. MgQf6 would be bound almost purely ionically, but it would be gaseous - Mg cannot have a higher co-ordination number than 6 so this cluster of ions cannot polymerise.

Therefore the polarity of the bonds in substances such as MoF6 and WF6 should have little effect on their volatility.

[enahs]

Silicon is less electronegative then carbon, and due to its larger size, the Si atom is not as well shielded as the Carbon atom.

[shelanachium]

How does that make SiCl4 boil lower than CCl4?