Which one is more stable OF6 and SF6?

Answer 1

A good question.

As with any problem-solving, let's start with what we already know.

O = 1s2 2s2 2p4

S = 1s2 2s2 2p6 3s2 3p4

They both have 6 valence electrons so I guess it's fair to assume they COULD both be 6 coordinate. It certainly explains why they have similar chemistry in some cases.

What's different about them, though?

-Sulphur is bigger than Oxygen

-Oxygen is more electronegative

-Sulphur's valence electrons are further from the nucleus and more shielded. (As it is in the 3rd shell rather than the 2nd like Oxygen)

All of these differences give answers to our question.

-Sulphur is bigger. You can fit more around it. It sounds too simplistic to be true but that's usually the best solution and actually this will be the major deciding factor. Getting 6 atoms around oxygen will be severely crowded, they'll repel each other and it won't be energetically favourable. We call this "Steric Hindrance" and it's the answer to a LOT of chemistry's problems.

- Oxygen is more electronegative. It doesn't like to give electrons away as much as sulphur does...and although fluorine is way more electronegative than either of them...by the time you oxidise oxygen a few times, it's really strongly hanging onto it's electrons. You won't take more than a couple from it.

Note, electronegativity is directly a product of atomic radius and nuclear charge, shielding and the like, it's not some strange standalone effect.

- Sulphur's valence electrons are further away from the nucleus and more shielded. By shielded we basically mean that there are more electrons between the outer electron and the nucleus (we know that's true, oxygen only has 1s2 whereas sulphur has 1s2, 2s2, 2p6 to help shield). If we shield our electrons from the nucleus, there's less positive charge pulling them in - meaning they're easier to pull off and bond with.

Being further away from the nucleus is helpful in the same way...although it's not really the same effect...they're similar enough to mention together.

Hope that's useful.

What are you smoking... Draw the Lewis dot diagram for SF6. 48 electrons to account for. Now place them. The only way to draw it correctly has 12 electrons associated with sulfur. We know that this is only possible for elements with n greater than or equal to 3. That's where d-orbitals become available. Oxygen is n=2 and has no d orbitals available, and we can't associate more than 8 electrons with it. Not because it would be "too crowded since oxygen is small". Yikes. It is not ok to assume oxygen can be greater than 4 coordinate, ever! Even OF4 is impossible; there are 2 electrons we can't account for when trying a Lewis dot structure.

This is the actual reason SF6 is possible.

Answer 2

SF6 is the only stable molecule among the two. Oxygen has no vacant orbitals to bond to 6 F atoms. So it does not form OF6 but S has vacant 3d orbitals and thus it shows the oxidation state of +6 and SF6 exists and is stable.