Why is sulfur hexafluoride more stable than selenium or tellurium hexafluoride?

Question

I was just reading about p Block elements from two different books. Both books say that $\ce{SF6}$ is extremely stable in 16th group due to steric reasons but my question is why is it more stable than $\ce{SeF6}$ and $\ce{TeF6}$ when $\ce{S}$ is more electronegative and smaller than $\ce{Te}$ and $\ce{Se}$ and so it would bring the electrons in bonding with fluorine closer to the central atom i.e $\ce{S}$ and thus it would bring the electrons closer and there would be more repulsion between the electron pairs.

Maybe this can be explained by Fajans rules that due to such conditions $\ce{SF6}$ has more covalent character than others and thus is more stable. But if this is the reason, where does "steric" come in here?

Answer 1

$\ce{SF6}$ is extremely stable for purely steric reasons, because S is completely blocked by fluorine atoms from all directions, so the reactions starting with an attack on S that otherwise would readily occur (hydrolysis, etc.) never have the chance to occur. This has nothing to do with electronegativity. For another similar example, look at $\ce{CCl4}$.

Now, the word "stable" is commonly used in two different senses. A compound may be stable by itself, i.e. not prone to spontaneous decomposition; in that sense $\ce{SF6}$, $\ce{SeF6}$, and $\ce{TeF6}$ are all totally stable under normal conditions, so there is nothing to compare. On the other hand, we may consider "stability" as inability to react with common reagents, e.g. water. In that sense $\ce{SF6}$ is indeed far more "stable" than the analogs, and that for the steric reasons (see above).

As for the thermal stability, that's another story altogether, so I don't really know how it would turn out. I wouldn't be surprised if $\ce{TeF6}$ would indeed happen to be the most stable of them all.