Why is `SF_6` much less reactive than `SF_4` ?

Explanation

In `SF_6` S is sterically protected by six F atoms and hence does not allow `H_2 O` molecules to attack the S atom. Further more, F does not have d-orbitals to accept the electrons donated by `H_2 O` moleucles. As a result of these two reasons, `SF_6` does not undergo hydrolysis. In contrast, in `SF_4`, S is not sterically protected since it is surrounded by only four F atoms. As a result, attack of `H_2 O` molecules on S atom can take palce easily and hence hydrolysis occurs.