`SF_(6)` is exceptionally stable due to

Which of the following compounds is exceptionally stable for steric reasons?

The +3 oxidation states of lanthanum(z=57), gadolinium (z=64) and lutetium (z=71) are exceptionally stable. Why?

Explain why : SF_(6) is inert and stable but SF_(4) is reactive.

Exceptionally Stable Carbocations Are|Summary

Why is SF_(6) kinetically inert ?