Why SF6 is inert while SF4 is a very reactive molecule that reacts with H2O rapidly and vigorously?

 

The fact that SF₆ does not react with water is not due to thermodynamic stability. Rather, it is because there is no low-energy pathway for the reaction to take place (kinetic stability). Six fluorine atoms surrounding the sulfur atom effectively prevent attack of water molecules and the sulfur atom has no unshared pairs of electrons where other molecules might attack. In SF₄ , not only is there sufficient space for an attacking species (water) to gain access to the sulfur atom, but also the unshared pair is a reactive site. As a result of these structural differences, SF₆ is relatively inert, whereas SF₄ is very reactive.

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