Why SF6 is inert where as SF4 is highly reactive towards H20 ?.

The maximum coordination number of sulphur is 6. Hence S cannot accommodate more than 12 electrons around it. SF₆ is covalently saturated compound. Thus six fluorine atoms around sulphur protect sulphur from attack by reagent to such an extent that even thermodynamically favorable reaction like hydrolysis (S-O bond is stronger than S-F bond) does not occur. On the other hand, in SF₄, the coordination number is only four. Hence sulphur can be attacked by H₂O.

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