Why SF4 is highly reactive and SF6 highly inert ?

In SF4, there are vacant d–orbital present as S is bonded to only 4 F atoms and hence has 1 non-bonding pair of electrons. Therefore SF4 is not stable and can react further with available non-bonding pair of electrons hence more reactive. While in SF6, there are no vacant d-orbital as the S is bonded with 6 F atoms and all the electrons are paired thus the octet of sulphur is complete making SF6 more stable and inert for further reactions.

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SF6 is stable due to steric causes

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In SF6,S is sterically protected by six F atoms and hence does not allow H2O molecules to attack the S atom.Furthermore,F does not havr d-orbitals to accept the  electrons donated by H2O molecules.As a result of these two reasons,SF6 does not undergo hydrolysis.In contrast,In SF4,S is not sterically protected since it is surrounded by only four atoms.As a result,attack of H2O molecules on S atom can take place easily and hence hydrolysis occurs.

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